Quandaries and Queries


Thank you,
parent, 7th grader



I have two responses, one from Leeanne and another from Denis.

Hi Lisa,

There is no one single rule for divisibility by 15, but every number that is divisible by 15 must be divisible by the factors of 15, namely 3 and 5. If your son knows these divisibility rules, then for divisibility by 15, he just includes both conditions.

If you are not sure of these 2 divisibilty rules, they are:

for divisibility by 3, add the digits of the number and if this sum is divisible by 3 then the number is divisible by 3

for divisibility by 5, the number must end in a 5 or a 0.

Hope this helps,

Hi Lisa, ,

To be divisible by 15 a number has to be divisible by 3 and by 5. To be divisible by 5 the number must end in a 0 or a 5, easy enough. However divisibility by 3 is not so simple - the rule is to add up all of the digits of the number and if they are a multiple of 3 then the original number is also. For example, 9273 is a multiple of 3 because 9+2+7+3 is a multiple of 3 while 9270 is not a multiple of 3 since 9+2+7+0 is not. So, is 85721445 a multiple of 15? Yes.



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