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.
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.
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 $6$?
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.
- a) How many stickers will each person receive? Are any left over?
- b) Is $28$ divisible by $4$?
- c) Is $4$ a factor of $28$?
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 $0$.
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 $0$.
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.
- a) Write down all the factors of $48$.
- 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.
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.
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:
- a) The number $60$ is a ____ of $15$.
- b) The number $4$ is a ____ of $60$.
- c) The number $60$ is a ____ of $4$.
- d) The number $15$ is a ____ of $60$.
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 $3$:
- a) $123$
- b) $401$
- c) $516$
- d) $802$
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 $9$.
- a) $2A5$
- b) $71A$
- c) $A08$
Rules for 4 and 11
Two more rules, a little less well known but just as useful.
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 $4$:
$1996, 2000, 2010, 2022, 2024, 2030$
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:
- a) $1.232$
- b) $5.049$
- c) $8.175$
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.
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).
- a) Find all possible ways to arrange the icons (for example: $1$ row of $30$).
- b) What mathematical relationship do the dimensions of these rectangles have with the number $30$?
- 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?
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:
- a) $24A$ is divisible by $4$ and $3$ at the same time.
- b) $51A$ is divisible by $11$.
- c) $A72$ is divisible by $9$ but not by $10$.
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.
- a) How many cookies have they baked exactly?
- b) Justify your answer using the divisibility tests for $2$ and $3$ to explain why the number is divisible by $6$.
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 $0$).