11. Given that $A$ and $B$ are subsets of the universal set $I$, and the number of elements in $A$ and $B$ are 18 and 10 respectively, and $I$ has 25 elements, find (1) the maximum and minimum number of elements in $A \cup B$; (2) the maximum and minimum number of elements in $A \cap B$.
11. (1) $A \cup B$ contains at most 25 elements, and at least 18 elements;
(2) $A \cap B$ contains at most 10 elements, and at least 3 elements.

From $|A \cup B|=|A|+|B|-|A \cap B|=18+10-|A \cap B|=28-|A \cap B|$, so $18 \leqslant$ $28-|A \cap B| \leqslant 25$, we get $3 \leqslant|A \cap B| \leqslant 10$.