Question

How many $3$-digit numbers have exactly one of their digits as $5$ $?$

Answer 1

How many $3$-digits numbers have exactly are of their digits as $5$

If $5$ is on first position

ways to select other digits $\to 5$_ _

$9\times 9=81$

If $5$ is on second position

ways to select other digits $\to$_$5$_

$8\times 9=72$

If $5$ is on third position

ways to select other digits $\to$ _ _ $5$

$8\times 9=72$

Total numbers=Total ways=$81+72+72=225$