Question

$3x5$ is divisible by $9$ if the digit $x$ is __________.

Answer 1

It is known that, if a number is a multiple of $9$, then the sum of digits of a number is divisible by $9$.

$3+x+5=8+x$ must be a multiple of $9$

The multiple can’t be $18$ or any other higher multiple of $9$, because then $x$ can't be a double-digit number.

$\begin{array}{rcl}\therefore x+8& =& 9\\ x& =& 9-8\\ & =& 1\end{array}$

Hence, the value of $x$ is $1$.