Word Problems with Equations
Yaritza and her two friends pool their money to buy a gift, but each one puts in a different amount. How can we find out how much each one gave? Solving the equation is the mechanical part, the one you already know how to do. The hard part (and what really matters) is setting it up: turning a statement into an equality with an $x$. With a clear method and a few models, most problems become routine.
The method in steps
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Choose the unknown and write down what it means: "$x$ = number of songs Wilkin has." Don't stop at the letter; note which number it refers to.
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Translate each piece of information in the sentence using that same $x$.
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Set up the equation with the missing piece of information (usually a total or an equality) and solve it.
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Check against the problem statement, not the equation (it might be set up wrong), and answer with a sentence and units.
If you add a consecutive number (the number that comes right after) to a number, the result is $43$. Set up an equation to find out what the number is.
If you add a consecutive number (the number that comes right after) to a number, the result is $43$. Set up an equation to find out what the number is.
A group of friends buys a family-sized pizza that costs $720$ pesos. If they have $480$ pesos left over after paying for it from the money they had collected, how much…
A group of friends buys a family-sized pizza that costs $720$ pesos. If they have $480$ pesos left over after paying for it from the money they had collected, how much money did they have at the start? Represent the initial amount with the letter $x$.
Numbers, "twice as many as," and ages
Two types of information come up again and again:
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Consecutive numbers: if the first is $x$, the next is $x + 1$ and the one after is $x + 2$.
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"Twice/three times as much as another quantity": if Enmanuel has $x$ songs and Yohanna has twice as many, Yohanna has $2x$. If Raysa also has $10$ more than Enmanuel, Raysa has $x + 10$.
For ages, it helps to make a small table with "today" and "in so many years": time passes the same for everyone, so the same amount is added to each age.
| Today | In 8 years | |
|---|---|---|
| Pamela | $x$ | $x + 8$ |
| Yeison | $x + 5$ | $x + 13$ |
When one person's age is three times another's, the equation comes from comparing the two rows of the table. For example, if the mother is $3x$ today and $3x + 12$ in 12 years, and her daughter is $x$ and $x + 12$, then "the mother will be twice as old" is written $3x + 12 = 2(x + 12)$. Here the parentheses are necessary: the doubling applies to the daughter's entire age.
Amina is $3$ years older than her younger brother. If their combined age is $19$ years, how old is each of them? Let $x$ be the age of the younger brother.
Amina is $3$ years older than her younger brother. If their combined age is $19$ years, how old is each of them? Let $x$ be the age of the younger brother.
Marcos is playing a video game and has scored twice as many points in the current level as in the previous level. If he scores a total of $150$ points across both…
Marcos is playing a video game and has scored twice as many points in the current level as in the previous level. If he scores a total of $150$ points across both levels, how many points did he get in each level? Let $x$ be the points in the first level.
Three friends are comparing how many songs they have in their favorite playlist. Lucía has twice as many songs as Marcos, and Sara has 15 more songs than Marcos. If all…
Three friends are comparing how many songs they have in their favorite playlist. Lucía has twice as many songs as Marcos, and Sara has 15 more songs than Marcos. If all three have a total of 155 songs altogether, how many songs does each person have in their playlist? Set up an equation where $x$ is the number of Marcos's songs.
Héctor is 5 years older than his sister Ainhoa. In 8 years' time, the sum of their ages will be 41 years. How old is each of them now? (Remember to add 8 years to each…
Héctor is 5 years older than his sister Ainhoa. In 8 years' time, the sum of their ages will be 41 years. How old is each of them now?
(Remember to add 8 years to each person's age to set up the future sum).
Salma's mother is currently three times her age. In $12$ years, her mother's age will be only twice Salma's age. How old is each of them currently? Use a table to…
Salma's mother is currently three times her age. In $12$ years, her mother's age will be only twice Salma's age. How old is each of them currently? Use a table to organise the data for the present and the future.
Balance scales and parts of a total
An equation is like a balance scale in equilibrium: what is on one pan weighs the same as what is on the other. If one pan has $x$ and a $12$ g weight, and the other has $4$ stones just like the first, we write $x + 12 = 4x$.
Other problems talk about two kinds of objects (cars and motorcycles, or coins of two values). If the total number of objects is $32$ and you call $x$ the number of one kind, the number of the other kind is $32 - x$. Then you count the quantity that tells them apart (wheels, money…).
The same goes for money: if there are $x$ coins worth $\text{RD}\$10$ and $30 - x$ worth $\text{RD}\$5$, the total value is $10x + 5(30 - x)$.
And when a problem talks about fractions, remember that what is left is calculated from what there was. If $\frac{1}{4}$ of a total $x$ is spent, what remains is $x - \frac{x}{4} = \frac{3x}{4}$. If $\frac{2}{5}$ of what remains is then spent, $\frac{3}{5}$ of $\frac{3x}{4}$ remains. You set it up by equating that to the final piece of information and solving for $x$.
In the Science lab we have a balance in equilibrium, as the figure shows. On the left pan there are $2$ identical boxes (each weighing $x$ kg) and a $1$ kg weight. On…
In the Science lab we have a balance in equilibrium, as the figure shows. On the left pan there are $2$ identical boxes (each weighing $x$ kg) and a $1$ kg weight. On the right pan there is a $7$ kg weight. Write the equation that represents this equilibrium and calculate how much each box weighs.
To balance a scale in the Science lab, we have placed a stone of unknown weight $x$ and a $12$-gram weight on the left pan. On the right pan, to keep it balanced, we had…
To balance a scale in the Science lab, we have placed a stone of unknown weight $x$ and a $12$-gram weight on the left pan. On the right pan, to keep it balanced, we had to put four equal weights, each with the same weight $x$ as the original stone. How much does the stone weigh?
In a shopping center parking lot there is a total of $32$ vehicles, including cars and motorcycles. If we count all their wheels (excluding spare tires), we get a total…
In a shopping center parking lot there is a total of $32$ vehicles, including cars and motorcycles. If we count all their wheels (excluding spare tires), we get a total of $94$ wheels. Calculate how many cars and how many motorcycles are in the parking lot.
(Hint: If we let $x$ be the number of cars, the number of motorcycles will be $32 - x$).
On a scientific expedition through the Amazon, the team consumes $\frac{1}{4}$ of their water supply on the first day. On the second day, they consume $\frac{2}{5}$ of…
On a scientific expedition through the Amazon, the team consumes $\frac{1}{4}$ of their water supply on the first day. On the second day, they consume $\frac{2}{5}$ of what was left in the tank. If after these two days they still have $135$ liters, how many liters of water did they have at the start of the expedition?
Problems with perimeters
In geometry, always draw the figure and write the measurements using $x$. Then use the perimeter formula (the sum of all the sides).
In an isosceles triangle, the two equal sides are written the same way. If the base measures $10$ cm and the equal sides measure $x$, the perimeter is $x + x + 10$.
We are designing an esports team logo shaped like an isosceles triangle. We want the perimetre to be $30$ cm. If the base measures $10$ cm and the other two sides are…
We are designing an esports team logo shaped like an isosceles triangle. We want the perimetre to be $30$ cm. If the base measures $10$ cm and the other two sides are equal, how long must each of those sides ($x$) be?
We want to fence off a small rectangular vegetable patch to plant tomatoes. The length of the patch is twice its width. If we have used $18$ meters of fencing to…
We want to fence off a small rectangular vegetable patch to plant tomatoes. The length of the patch is twice its width. If we have used $18$ meters of fencing to completely surround it (perimeter), what are the width and length of the patch?
At Adrián's birthday party, two types of invitations were handed out: digital and paper. The number of digital invitations was three times the number of paper ones. If a…
At Adrián's birthday party, two types of invitations were handed out: digital and paper. The number of digital invitations was three times the number of paper ones. If a total of 48 invitations were sent out, how many people received a paper invitation and how many a digital one?
The perimeter of an isosceles triangle-shaped technology park is $110$ meters. We know that each of the equal sides measures $10$ meters more than twice the base.…
The perimeter of an isosceles triangle-shaped technology park is $110$ meters. We know that each of the equal sides measures $10$ meters more than twice the base. Calculate the length of each of the three sides of the park by setting up an equation.
Sharing and related quantities
When several people split a cost, or when several quantities are related to each other, call $x$ the share of one of them and write the others in terms of $x$. The equation comes from the fact that all the parts together make up the total.
The same works with a store's sales, video views, or any quantity that depends on another: choose as $x$ the one that serves as the base for the others.
Back to the beginning: with the method, Yaritza and her friends' gift is solved the same way as the siblings' gift: one share is $x$, the others are written using $x$, and all of them add up to the total.
On a shelf in a comic book shop, there are three times as many action manga as mystery manga. If there are $48$ comics in total between both genres, how many are there…
On a shelf in a comic book shop, there are three times as many action manga as mystery manga. If there are $48$ comics in total between both genres, how many are there of each type? Let $x$ be the mystery comics.
Marta bought a video game on offer and a new controller. The controller cost her $1\,500$ pesos less than the video game. If she paid a total of $5\,340$ pesos for both…
Marta bought a video game on offer and a new controller. The controller cost her $1\,500$ pesos less than the video game. If she paid a total of $5\,340$ pesos for both items, what is the original price of each item without counting other discounts?
The PE teacher has bought three boxes of tennis balls. The second box contains twice as many balls as the first, and the third box has $10$ more balls than the second.…
The PE teacher has bought three boxes of tennis balls. The second box contains twice as many balls as the first, and the third box has $10$ more balls than the second. If there are $85$ balls in total, how many balls are in each box?
In a games shop, they sold three times as many copies of an adventure game as a sports game. Furthermore, they sold $15$ fewer units of a platform game than the…
In a games shop, they sold three times as many copies of an adventure game as a sports game. Furthermore, they sold $15$ fewer units of a platform game than the adventure game. If they sold a total of $125$ video games across the three titles, how many copies were sold of each one?
A digital content creator reviewed their monthly statistics. The comedy video has twice as many views as the technology video, and the travel video has $4000$ more views…
A digital content creator reviewed their monthly statistics. The comedy video has twice as many views as the technology video, and the travel video has $4000$ more views than the comedy video. If all three videos total $24000$ views altogether, how many views does each video have?
In a piggy bank, there are $0.50$ pesos and $1$ peso coins. In total, there are $35$ coins with a total value of $26$ pesos. How many coins of each type are in the piggy…
In a piggy bank, there are $0.50$ pesos and $1$ peso coins. In total, there are $35$ coins with a total value of $26$ pesos. How many coins of each type are in the piggy bank? (Hint: if you let $x$ be the number of $1$ peso coins, the number of $0.50$ pesos coins will be $35 - x$).
Three siblings, Alberto, Berta and Carlos, want to buy a present for their parents that costs $105$ pesos. Alberto contributes an amount of money, Berta gives twice as…
Three siblings, Alberto, Berta and Carlos, want to buy a present for their parents that costs $105$ pesos. Alberto contributes an amount of money, Berta gives twice as much as Alberto, and Carlos gives $15$ pesos less than Berta. How much money does each person contribute so that they have exactly enough for the gift?