How do you beat Safe Cracking for mission 4 in the 39 clues?

Okay, well, it's kind of like memory or concentration. Let's say you click on a card; it flips over, and it's a plug-in thing. Then you click on another one; it's an atom. They flip back over, because they don't match.

But if you click on cards that match, they will be cleared off the screen. You have to clear all of them before time runs out, because each card that hasn't been cleared will be -25 point.