Show Hint (added 2006-08-14)
One of the ways my original program ranked difficulty is that multiplication and addition are considered "easy," but division and subtraction were considered "hard." What operations do you think this "hard" puzzle would have?
Addendum (2006-08-04): People often assume that I didn't create this puzzle, perhaps because it seems so "classic." In fact, I did create it, back in 2001, when I wrote a program to solve all sets of four digits -- and rank the resulting puzzles all by difficulty.