36. Multiply your shoe size by 2, add 39 to the product, multiply the resulting sum by 50, add 40 to the product, and subtract your birth year from the sum. You will get a four-digit number, the first two digits of which are your shoe size, and the last two digits are your age at the end of the calendar year 1990. Explain why. (Try this trick on your friends.)
36. Let the size of your shoe be $\overline{a b}$ (a two-digit number). Perform the following steps sequentially:
1) $\bar{a} \bar{b} \cdot 2 ; 2) 2 \cdot \overline{a b}+39$
2) $(2 \overline{a b}+39) \cdot 50=100 \cdot \overline{a b}+1950$;
3) $100 \cdot \bar{a} \bar{b}+1950+40=100 \overline{a b}+1990$.

If the birth year is 1975, then $1990-1975=15$. We get the number $100 \overline{a b}+15$, which is a four-digit number where the first two digits form the number $\overline{a b}$ (shoe size), and the last two digits are 15 (age).