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 $9$ or $18$ or ...

Since $y$ is a digit, so $(8+y)$ must be equal to $9$.

i.e., $8+y=9$

$\Rightarrow y=9-8=1$