Using the numbers 3, 3, 8, 8 (in any order), make a mathematical expression that equals 24. You can use only addition, subtraction, multiplication, and division (and parentheses), but in any order you wish. Note that you have to use all four numbers; otherwise 3 times 8 would be valid -- and that wouldn't be much of a puzzle, would it?
It took a little while, but I figured it out. Here's a link to a Google Calculator version of the solution.