7. When asked how old he was, a mathematician replied: "If you subtract 5 from my age, divide the result by 5, and then subtract 5 again, you will get the number 5." How old is he?
7. Solving backwards.

$5+5=10$.
$10 \cdot 5=50$. ..... 3 points
$50+5=55$. ..... 3 points
The mathematician is 55 years old. ..... 1 point