EXERCISE

If $21y5$ is a multiple of $9$, where $y$ is a digit. What is the value of $y$?

If $31\mathrm{z}5$ is a multiple of $9$, where $\mathrm{z}$ is a digit, what is the sum of all possible values of $\mathrm{z}$?

$24x$ is a multiple of $3$. If $x$ is a digit, what is the sum of all possible values of $x$?

If $31\mathrm{z}5$ is a multiple of $3$, where $\mathrm{z}$ is a digit, what will be the values of $\mathrm{z}$. $(2, 3, 8, 7)$