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ESO Y1 · Mathematics
Numbers · Divisibility

Multiples, divisors and divisibility rules

A grasshopper always jumps $6$ decimetres at a time. After several jumps, can it land exactly on the $20$ decimetre mark? And if you share $28$ stickers among $4$ friends, are any left over? These questions are about multiples and divisors: the foundation of everything that comes next in this topic.

Multiples and divisors

$6, 12, 18, 24, 30\ldots$ are the grasshopper's jumps: each one is $6$ times a natural number. They are called multiples of 6.

Animation of a number line in which a grasshopper (a dot) makes arcing jumps of 6 from 0 to 24; each arc is drawn with its +6 while the dot is in the air and, when it lands, the label of the multiple of 6 appears. jumps of 6 +6 +6 +6 +6 06121824 6 · 0 6 · 1 6 · 2 6 · 3 6 · 4

Notice: "$60$ is a multiple of $15$", "$15$ is a divisor of $60$" and "$60$ is divisible by $15$" all say exactly the same thing; only the point of view changes.

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A grasshopper is training for a competition. With each jump it travels forward exactly $6$ decimetres. Write down the total distance it has travelled after taking…

A grasshopper is training for a competition. With each jump it travels forward exactly $6$ decimetres.

Write down the total distance it has travelled after taking $1, 2, 3, 4, 5$ and $10$ jumps. What are these numbers called with respect to $6$?

1º 2º 3º 4º 5º 10º ... +6 +6 +6 +6 +6 +6 0 6 ... dm
Solve
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Marta wants to share $28$ stickers equally among her $4$ friends. a) How many stickers will each person receive? Are any left over? b) Is $28$ divisible by $4$? c) Is…

Marta wants to share $28$ stickers equally among her $4$ friends.

  1. a) How many stickers will each person receive? Are any left over?
  2. b) Is $28$ divisible by $4$?
  3. c) Is $4$ a factor of $28$?
28 pegatinas 4 amigos
Solve

Finding all the divisors

To find all the divisors of a number, you try each one in turn from $1$ up to the number itself and keep those that leave remainder $0$.

Diagram showing how the divisors of a number are obtained, organised in pairs of complementary factors. Divisors of 24: 1 2 3 4 6 8 12 24 (5, 7, 9... are not included: they leave a remainder)
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Identify which of the following numbers are factors of $24$ by performing the necessary divisions: $$2, 5, 6, 9, 12, 14$$ Remember: A number is a factor of another if,…

Identify which of the following numbers are factors of $24$ by performing the necessary divisions:

$$2, 5, 6, 9, 12, 14$$

Remember: A number is a factor of another if, when divided, the remainder is equal to $0$.

Solve
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An animal shelter has received a donation of $48$ cans of cat food. They want to store them in small boxes so that all the boxes have the same number of cans and none…

An animal shelter has received a donation of $48$ cans of cat food. They want to store them in small boxes so that all the boxes have the same number of cans and none are left over.

  1. a) Write down all the factors of $48$.
  2. b) If the manager wants each box to contain more than $5$ cans but fewer than $15$, what packaging options do they have? State the number of cans per box and how many boxes would be needed in each case.
Solve

Divisibility rules: 2, 3, 5, 9 and 10

Looking only at the digits, without dividing, tells you instantly whether a number is a multiple of another.

Visual example of the rule for 3 comparing the number 516 (its digits add up to 12, divisible) with 401 (its digits add up to 5, not divisible). 5165+1+6 = 12 12 is a multiple of 3→ 516 is divisible by 3 4014+0+1 = 5 5 is not a multiple of 3→ 401 is NOT divisible by 3
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Knowing that the equality $15 \cdot 4 = 60$ is true, complete the following sentences using the words multiple or factor correctly: a) The number $60$ is a of $15$. b)…

Knowing that the equality $15 \cdot 4 = 60$ is true, complete the following sentences using the words multiple or factor correctly:

  1. a) The number $60$ is a ____ of $15$.
  2. b) The number $4$ is a ____ of $60$.
  3. c) The number $60$ is a ____ of $4$.
  4. d) The number $15$ is a ____ of $60$.
Solve
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A librarian has to place books on shelves where exactly $3$ books fit into each space. To quickly find out if they can fill the shelves without any books left over, they…

A librarian has to place books on shelves where exactly $3$ books fit into each space. To quickly find out if they can fill the shelves without any books left over, they use the divisibility test for $3$.

Calculate the sum of the digits of the following numbers and state which ones are divisible by $3$:

  1. a) $123$
  2. b) $401$
  3. c) $516$
  4. d) $802$
Solve
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Replace the letter $A$ with a digit so that the resulting number is a multiple of $9$. Remember that the sum of its digits must be a multiple of $9$. a) $2A5$ b) $71A$…

Replace the letter $A$ with a digit so that the resulting number is a multiple of $9$. Remember that the sum of its digits must be a multiple of $9$.

  1. a) $2A5$
  2. b) $71A$
  3. c) $A08$
Solve

Rules for 4 and 11

Two more rules, a little less well known but just as useful.

Visual diagram explaining the divisibility rule for 11 by adding the digits in even and odd positions separately. 2024last two digits: 24 24 : 4 = 6→ divisible by 4 5.049(5+4) − (0+9) = 9 − 9 = 0 0→ divisible by 11
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A number is divisible by $4$ if its last two digits are zeros or form a multiple of $4$. Look at the following years and say which ones would be "special years" because…

A number is divisible by $4$ if its last two digits are zeros or form a multiple of $4$.

Look at the following years and say which ones would be "special years" because they are divisible by $4$:

$1996, 2000, 2010, 2022, 2024, 2030$

Solve
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Apply the divisibility test for $11$ to determine which of the following numbers are multiples of $11$. For those that are not, state how much you should add to them so…

Apply the divisibility test for $11$ to determine which of the following numbers are multiples of $11$. For those that are not, state how much you should add to them so that they are:

  1. a) $1.232$
  2. b) $5.049$
  3. c) $8.175$
+ + d₁ d₂ d₃ d₄ (d₁ + d₃) - (d₂ + d₄) = 0, ±11, ±22... Odd positions Even positions
Solve

Combining several rules

The most useful approach is to chain rules together: if a number meets two at once, you have ruled out a lot of candidates in one go.

Diagram of nested rectangles showing how to filter numbers that meet the rules for 2, 3 and 9 at the same time. all the multiples of 2 ...that are also multiples of 3 → multiples of 6 ...and of 9 → the solution
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A graphic designer is arranging a gallery of $30$ identical square icons for a website. They want to arrange them into a complete rectangle (without any icons left…

A graphic designer is arranging a gallery of $30$ identical square icons for a website. They want to arrange them into a complete rectangle (without any icons left over).

  1. a) Find all possible ways to arrange the icons (for example: $1$ row of $30$).
  2. b) What mathematical relationship do the dimensions of these rectangles have with the number $30$?
  3. c) If the design only allows the shorter side of the rectangle to have at least $3$ icons, how many options do they have now?
Solve
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Find the value of the missing digit, represented by the letter $A$, in each of these numbers so that the given condition is satisfied: a) $24A$ is divisible by $4$ and…

Find the value of the missing digit, represented by the letter $A$, in each of these numbers so that the given condition is satisfied:

  1. a) $24A$ is divisible by $4$ and $3$ at the same time.
  2. b) $51A$ is divisible by $11$.
  3. c) $A72$ is divisible by $9$ but not by $10$.
a) 2 4 A ÷ 3 and ÷ 4 b) 5 1 A ÷ 11 c) A 7 2 ÷ 9 and ∤ 10
Solve
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A baker has baked a tray of cookies. They know they have more than $70$ cookies but fewer than $85$. If they pack them in boxes of $6$, none are left over. If they pack…

A baker has baked a tray of cookies. They know they have more than $70$ cookies but fewer than $85$. If they pack them in boxes of $6$, none are left over. If they pack them in boxes of $9$, none are left over either.

  1. a) How many cookies have they baked exactly?
  2. b) Justify your answer using the divisibility tests for $2$ and $3$ to explain why the number is divisible by $6$.
Solve

Back to the start: The grasshopper cannot land on $20$ because $20$ is not a multiple of $6$ ($20 : 6 = 3$, remainder $2$); on the other hand, the $28$ stickers can be shared exactly among $4$ friends because $28$ is a multiple of $4$ ($28 : 4 = 7$, remainder $0$).

Summary

See all the exercises in this section