Question

If 21y5 is multiple of 9, where y is a digit, what is the value of y.

Answer 1

Since the number 21y5 is a multiple of 9.
So, the sum of its digits 2+1+y+5=8+y is a multiple of 9.
$\therefore$ (8+y) is either 0 or 9 or 18 or ...
Since y is a digit, so (8+y) must be equal to 9. i.e., 8+y=9 $\Rightarrow y=9-8=1$