<ol>
<li>There are 25 trays on a table in the cafeteria. Each tray
contains a cup only, a plate only, or both a cup and a
plate. If 15 of the trays contain cups and 21 of the trays
contain plates, how many contain both a cup and a
plate?</li>
</ol>
<p>Inclusion-exclusion. 25 = 15 + 21 - x, where x is the number of tests containing both a cup and a plate. x = 11, so 11 trays have both a cup and a plate.</p>