963. Alyosha says to Borya: «Think of a two-digit number. Multiply it by 2, add 5 to the product, and then multiply the resulting sum by 5. What did you get? 715? You thought of 69.» How did he guess?
$\triangle$ Let's denote the number thought of by Borey as $\overline{a b}$. We will perform all the specified actions.

$$
(\overline{a b} \cdot 2+5) \cdot 5=\overline{a b} \cdot 10+25
$$

To get to the number $\overline{a b}$, first, we discard the digit 5 from the obtained number; it will turn into the number $\overline{a b}+2$. Then, we need to subtract 2 from the last number.

Thus, for Aleksey to find the thought-of number, he needs to mentally perform the following: in the number reported to him, discard the last digit 5 (which is always 5) and subtract 2 from the resulting number.