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Math 6 · Mathematics
Numbers · Integers

Problems with Integers

Jayden has $\$30$ in his gaming wallet and wants a $\$45$ game. His mom says she'll cover the rest and he can pay her back later. How do you write what he has now? Bank balances, thermometers, elevators and calendar years all work the same way: there is a starting point and a series of moves that go up (positive) or down (negative).

Money and balances

Balance diagram: a starting balance of 40 dollars and three moves of minus 18, plus 10 and minus 35 give a final balance of minus 3 dollars. Start: $40 −$18 +$10 −$35 40 − 18 + 10 − 35 = −$3

Two facts about signs will keep coming up:

  • Adding a negative number is the same as subtracting: $5 + (-3) = 5 - 3$.

  • Subtracting a negative number is the same as adding: $5 - (-3) = 5 + 3$. Taking away a debt makes you richer.

When you earn and spend money, the profit is money in minus money out. A negative profit is a loss. Repeated moves (selling 10 items at the same price) are multiplication.

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Emma has $34\ \$$ in her piggy bank. She decides to buy a book that costs $15\ \$$ and a backpack that costs $25\ \$…

Emma has $34\ \$$ in her piggy bank. She decides to buy a book that costs $15\ \$$ and a backpack that costs $25\ \$$. As she does not have enough money, her parents tell her she can owe them what she is missing. What integer represents Emma's final balance with respect to her parents?

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Emma has $\$45$ in her bank account. During the weekend, she makes the following transactions: she pays a streaming platform subscription for $\$12$, receives a Zelle…

Emma has $\$45$ in her bank account. During the weekend, she makes the following transactions: she pays a streaming platform subscription for $\$12$, receives a Zelle from her father for $\$20$ and buys a book that costs her $\$15$. If at the end of Sunday her bank charges her a maintenance fee of $\$3$, what is the final balance of her account?

$0 $10 $20 $30 $40 $50 $60 +$45 -$12 +$20 -$15 -$3 ?$ Start Series Venmo Book Fee Balance
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A young entrepreneur decides to open an online personalized T-shirt shop. During his first month, he records the following movements in his bank account: He receives an…

A young entrepreneur decides to open an online personalized T-shirt shop. During his first month, he records the following movements in his bank account:

  • He receives an initial investment of $\$1.200$.

  • He pays the platform rent and advertising: $\$450$.

  • He buys materials worth $\$600$.

  • He sells $45$ T-shirts at $\$15$ each.

  • $8$ faulty T-shirts are returned, for which he has to refund the full amount to the customers.

  • At the end of the month, the bank charges him a fee of $\$12$.

Calculate the final balance for the month and explain whether he has made a profit or a loss compared to his initial investment.

Solve

Scores and repeated moves

In games, every action adds or subtracts a fixed number of points. When the same action happens several times, multiply first, then combine.

Trivia score: 7 correct answers worth 10 points each give plus 70, 4 wrong answers worth minus 5 each give minus 20, total 50 points. 7 × 10 = +70 4 × (−5) = −20 70 − 20 = 50 points

Some games also multiply or divide the score. By the order of operations, multiplication and division happen before addition and subtraction. To make an operation apply to everything that came before it, put that part in parentheses.

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In a video game tournament, the team "The Gamers" has a total of $-150$ points accumulated for having committed infractions. If in the last round they get a bonus of…

In a video game tournament, the team "The Gamers" has a total of $-150$ points accumulated for having committed infractions. If in the last round they get a bonus of $225$ points, how many points do they have now on their scoreboard?

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In a board game, you win $15$ points for each correct answer, but you lose $8$ points for each mistake and $4$ points for each question you leave unanswered. If out of a…

In a board game, you win $15$ points for each correct answer, but you lose $8$ points for each mistake and $4$ points for each question you leave unanswered. If out of a total of $20$ questions you got $12$ right, $5$ wrong, and left the rest unanswered, what is your final score?

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In a video game tournament, the scoring system works as follows: Each win adds $100$ points. Each loss subtracts $60$ points. Each "special achievement" multiplies the…

In a video game tournament, the scoring system works as follows:

  • Each win adds $100$ points.

  • Each loss subtracts $60$ points.

  • Each "special achievement" multiplies the accumulated score at that moment by $2$ (if it is positive).

  • Each penalty divides the accumulated score by $2$ (if it is positive).

If a player starts with $0$ points and gets: $3$ consecutive wins, a "special achievement", $2$ losses, a penalty and, finally, $1$ win:

  1. a) Write a single expression with combined operations that calculates their final score.
  2. b) What is the result of this score?
Solve

Temperature changes

A thermometer is a vertical number line. A rise is $+$, a drop is $-$. Here are the temperatures of a winter day in Denver: $6$, then $19$, then $-6$.

A steady drop is a rate. If the temperature changes by the same amount each hour, multiply the rate by the number of hours. To find how long something takes, find the total change needed and divide by the rate.

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At a ski resort in the Rocky Mountains, the thermometer showed $-5^{\circ}C$ at seven in the morning. At twelve noon, the temperature had gone up by $8^{\circ}C$, but by…

At a ski resort in the Rocky Mountains, the thermometer showed $-5^{\circ}C$ at seven in the morning. At twelve noon, the temperature had gone up by $8^{\circ}C$, but by midnight, it had gone down by $6^{\circ}C$ compared to the noon temperature. What temperature did the thermometer show at the end of the day?

T (°F) 10 5 0 −5 −10 +3 ? Time 07:00 12:00 24:00 +8°F −6°F
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A scientist is conducting a chemical experiment. He places a compound in a special freezer that lowers the temperature of the sample by $4^\circ C$ every hour. If the…

A scientist is conducting a chemical experiment. He places a compound in a special freezer that lowers the temperature of the sample by $4^\circ C$ every hour. If the sample was initially at a room temperature of $18^\circ C$, what will the temperature of the sample be after $6$ hours? Express the result using a single expression with integers.

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In a scientific laboratory, an experiment is being carried out with a gaseous substance. The initial temperature of the sample is $12$ ºF below zero. The process follows…

In a scientific laboratory, an experiment is being carried out with a gaseous substance. The initial temperature of the sample is $12$ ºF below zero. The process follows these phases:

  1. During the first $5$ minutes, the temperature drops by $3$ ºF every minute.

  2. Then, it is placed in a heat chamber where the temperature rises by $48$ ºF in total.

  3. Finally, the chamber is turned off and the temperature drops at a constant rate of $4$ ºF every minute until it reaches the freezing temperature of the sample, which is $-25$ ºF.

How long did the final cooling phase (point 3) last? What was the maximum temperature reached during the whole experiment?

Time Temp (°F) 30 20 10 0 −10 −20 −30 −12 °F −27 °F −25 °F Phase 1 −3 °F/min × 5 ΔT = −15 °F Phase 2 ΔT = +48 °F Phase 3 ΔT = −46 °F Rate: −4 °F/min
Solve

Elevation, depth and floors

Elevators, scuba divers and drones move up or down from a level called $0$ (the ground floor, or sea level). Above is positive and below is negative.

Vertical axis with sea level at 0 feet and a diver who starts at minus 20 feet, goes down to minus 35 feet and then up to minus 27 feet. sea level (0 ft) start: −20 ft up 8: −27 ft down 15: −35 ft
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A food delivery driver enters the elevator of a large shopping mall on the third basement floor, which is indicated as floor $-3$. First, he goes up $7$ floors to…

A food delivery driver enters the elevator of a large shopping mall on the third basement floor, which is indicated as floor $-3$. First, he goes up $7$ floors to deliver an order and, right after, he goes down $2$ floors to collect another order. On which floor is the delivery driver after these movements?

+5 +4 +3 +2 +1 0 −1 −2 −3 −4 Goes up 7 Goes down 2 Start End
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A diver is submerged in the ocean at a depth of $18$ feet below sea level ($-18\ m$). If he ascends $7$ feet to observe a coral reef and then descends again by $4$ feet,…

A diver is submerged in the ocean at a depth of $18$ feet below sea level ($-18\ m$). If he ascends $7$ feet to observe a coral reef and then descends again by $4$ feet, at what depth is he finally? Express the result as an integer.

Sea level (0 ft) 0 −10 −20 −18 +7 ft −4 ft ? ?
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A diver exploring a coral reef is $14$ feet below sea level. If he decides to descend another $18$ feet to observe some archaeological remains and then goes up $9$ feet…

A diver exploring a coral reef is $14$ feet below sea level. If he decides to descend another $18$ feet to observe some archaeological remains and then goes up $9$ feet to get closer to the surface, at what depth is he now? Represent the situation using integers.

Solve

Distance between two levels

A position can be negative, but a distance is never negative. To find the distance between two levels, subtract the lower one from the higher one: higher $-$ lower.

Vertical axis showing a hot-air balloon at plus 250 feet and a submarine at minus 60 feet, with the distance between them being 250 minus negative 60, which is 310 feet. balloon: +250 ft sea level (0 ft) submarine: −60 ft distance: 250 − (−60) = 310

The same idea tells you how far something has fallen or climbed: for a drop, distance $=$ start $-$ end. A drone that starts at $300$ ft and lands at $-20$ ft fell $300 - (-20) = 320$ ft.

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The highest peak of a volcanic island is at $1\ 200$ feet above sea level. On the same island, there is an underwater cave whose base is at $85$ feet below sea level.…

The highest peak of a volcanic island is at $1\ 200$ feet above sea level. On the same island, there is an underwater cave whose base is at $85$ feet below sea level. What is the difference in height between the top of the peak and the base of the cave?

Sea level Summit Cave base 1,200 ft 85 ft 0 ft ¿?
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The summit of Mount Everest is at an altitude of $+8.848$ feet above sea level, while the deepest point of the Mariana Trench is at $-11.034$ feet. What is the total…

The summit of Mount Everest is at an altitude of $+8.848$ feet above sea level, while the deepest point of the Mariana Trench is at $-11.034$ feet. What is the total vertical distance between the highest and lowest points on the planet?

+10000 +5000 −5000 −10000 −15000 Sea level (0 ft) +8,848 ft Everest −11,034 ft Mariana Trench ? ft Total distance
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A forest surveillance drone takes off from a base located at an altitude of $850$ feet. During its patrol, it makes the following movements: 1. It goes up $120$ feet to…

A forest surveillance drone takes off from a base located at an altitude of $850$ feet. During its patrol, it makes the following movements:

  1. It goes up $120$ feet to avoid a mountain.

  2. It goes down $300$ feet to analyze an area of low vegetation.

  3. It moves horizontally and then goes down another $150$ feet.

  4. Due to a technical failure, it falls to the bottom of a ravine which is at an altitude of $-20$ feet (below sea level).

Calculate how many feet the drone has descended in its final fall (step 4). If the drone were able to go up from the ravine directly to the initial base, how many feet would it have to ascend?

0 ft +800 ft +600 ft +400 ft +200 ft −200 ft −400 ft 1. Goes up 120 ft 2. Goes down 300 ft 3. Goes down 150 ft 4. Falls to bottom (?) Return (?)

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Solve

Years BC and AD

Years can go on a number line too. Treat years AD as positive and years BC as negative. The farther left a year is, the older it is.

Timeline with year 0 in the middle, 69 BC and 30 BC as negative numbers on the left and 79 AD as a positive number on the right. 0 69 BC−69 30 BC−30 79 AD+79 BC: negativeAD: positive

Questions like "born $x$ years before" or "at what age" are just addition and subtraction of years. Birth year $=$ death year $-$ age, and age at an event $=$ event year $-$ birth year.

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A historian is studying the life of a sculptor from Ancient Greece. According to the records, the sculptor was born in the year $495$ BC and died in the year $429$ BC.…

A historian is studying the life of a sculptor from Ancient Greece. According to the records, the sculptor was born in the year $495$ BC and died in the year $429$ BC. How many years did this sculptor live?

Solve
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Pythagoras, the famous Greek mathematician and philosopher, was born in the year $570$ BC and died in the year $495$ BC. At what age did Pythagoras die? Use integers…

Pythagoras, the famous Greek mathematician and philosopher, was born in the year $570$ BC and died in the year $495$ BC. At what age did Pythagoras die? Use integers (associating the years before Christ with negative numbers) to perform the calculation.

Towards year 0 ➔ −600 −550 −500 −570 Birth −495 Death Age?
Solve
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The Greek philosopher Hypatia of Alexandria died in the year $415$ AD at the age of $45$. Her father, Theon of Alexandria, was a famous mathematician who was born $80$…

The Greek philosopher Hypatia of Alexandria died in the year $415$ AD at the age of $45$. Her father, Theon of Alexandria, was a famous mathematician who was born $80$ years before she died.

  1. a) In what year was Hypatia born? (Express it as an integer).
  2. b) In what year was her father Theon born?
  3. c) How old was Theon when his daughter was born?
Solve

Multi-step problems

Long problems chain many moves. Write the position after each step so you never have to keep everything in your head.

Elevator trip: start at floor 0, up 8 to floor 8, down 11 to floor minus 3, up 5 to floor 2, down 10 to floor minus 8. start0 up 8+8 down 11−3 up 5+2 down 10 (2 × 5)final floor: −8

Two extra ideas show up in longer problems:

  • Total distance traveled counts every move as positive: $8 + 11 + 5 + 10 = 34$ floors. This is different from the net change (final position minus start), which here is $-8$.

  • Halfway between two numbers: find the gap, take half, and add it to the lower number. Halfway between $-8$ and $4$: the gap is $12$, half is $6$, and $-8 + 6 = -2$.

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An Amazon delivery driver starts his shift on floor $3$ of a large logistics center. First, he goes down $5$ floors to pick up a package in the basement, then he goes up…

An Amazon delivery driver starts his shift on floor $3$ of a large logistics center. First, he goes down $5$ floors to pick up a package in the basement, then he goes up $7$ floors to deliver it to an office and, finally, he goes down another $3$ floors to rest in the cafeteria. On which floor is the cafeteria? Express the result as an integer.

Solve
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An underwater cave explorer begins his dive from a platform located at $4$ feet above sea level. First, he descends at a constant speed of $2$ feet per minute for $15$…

An underwater cave explorer begins his dive from a platform located at $4$ feet above sea level.

  • First, he descends at a constant speed of $2$ feet per minute for $15$ minutes.

  • Then, he encounters an updraft that lifts him $12$ feet.

  • Finally, he decides to go down to the bottom of the cave, which is at a depth that is three times his current position.

At what depth is the bottom of the cave? Express the result as an integer relative to sea level.

10 −10 −20 −30 −40 −50 Sea level +4 0 −14 −26 −42 Descends −30 ft Ascends (updraft) +12 ft Descends to triple 3 × (−14 ft) +4 ft −26 ft −14 ft −42 ft
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A parcel delivery driver uses an elevator in a large logistics center that has floors above and below ground (floor $0$). He makes the following route: 1. He goes up…

A parcel delivery driver uses an elevator in a large logistics center that has floors above and below ground (floor $0$). He makes the following route:

  1. He goes up from floor $0$ to floor $+12$.

  2. He goes down $15$ floors to collect a package.

  3. He goes up $7$ floors to deliver it.

  4. He goes down twice as many floors as he has just gone up.

  5. Finally, he goes up until he is halfway between the floor he is on and floor $+8$.

On which floor is the delivery driver at the end of his shift? How many floors has he traveled in total (counting going up and down)?

0 20 15 10 5 −5 −10 −15 1. Goes up 12 2. Goes down 15 3. Goes up 7 4. Goes down 14 5. Goes up ? Final

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  <path d="M 0 0 L 8 4 L 0 8 z" fill="#10b981" />

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<circle cx="250" cy="286" r="3

Solve

Back to the beginning: Jayden has $\$30$ and spends $\$45$, so his balance is $30 - 45 = -15$. The $-15$ means he owes his mom $\$15$.

Summary

See all the exercises in this section