Task 1. Each member of a team plays football or tennis. How many people are in the team if it is known that 18 of them play both football and tennis, 23 play football, and 21 play tennis.
Solution. Let $E$ be the set of all team members, $F$ be the set of team members who play football, and $T$ be the set of team members who play tennis. Then,

$$
\delta F=23, \delta T=21, \delta(F \cap T)=18 . \quad \delta E=18+(23-18)+(21-18)=26
$$