Here's how you can use Spotlight in OS X on your Mac to do calculations, instead of opening the Calculator app.
1. Click the Spotlight icon in the menu bar.
2. Type in the mathematical expression you want to use.
3. Spotlight will show you the result.
4. Hit the Enter key after typing the expression to open the Calculator app.
For more, see the original article at the link below.
5 Essential Time-Saving Tips for OS X Mountain Lion | TechnoDuet