Spotlight doesn't solve math operations any more

open up a terminal and try

defaults write com.apple.spotlight CalculationEnabled YES

If that doesn't work, you might need to remove com.apple.Spotlight.plist from your user preferences folder, and then re-boot. This will remove any exclusions you have set up though, so you will need to manually put them back.

If that doesn't work, open up calculator and use it to perform a calculation. That as worked for some people.

There were some problems with this caused by the 10.5.6 update, so if you have recently upgraded, or the steps above don't work. You might want to download the combo update for you current version and reinstall it.

Installing Snow Leopard fixed the problem - hopefully it doesn't recur, since I never figured out what caused it in the first place!

Spotlight does this by "feeding" calculator.app. The Calculator program must be in a folder being checked by Spotlight in order for this function to work.

Make sure Spotlight can find Calculator.app by checking Spotlight Preferences.

Or perhaps you moved Calculator.app to another location, in that case you might just want to move it back to Applications.

P.S. Your problem is not uncommon, I've seen this question asked before. So +1 to your question.