Question

if 31y5 is a multiple of 9. where y is a digit, what is the value of y?

Answer 1

The sum of all digits of a number divisibe by 9 must be divisible by 9 itself.

Using this relation,

$\frac{3+1+y+5 }{9} = \frac{8 + y}{9}$

What is the one digit value you should put in y to make this whole thing divisible by 9?

The only answer is 1.

So now putting this value in the number abover we get 3115.

Tada!