There are several ways to add calculations (i.e.: formulas) to an Apple Numbers (part of the iWork suite) spreadsheet. The traditional method is to place the cursor in the field where you want the formula located, then begin entering the formula with the "=" sign. That's still a great method for the more complex formulas, but when it comes to simple formulas, there's a much (much) quicker method.
In this example, we want to generate a test score average for each student. To start, we need to select the four test scores that we want to average. You'll notice as we do this that Numbers has already generated some generic calculations, which are located in the lower left-hand corner of the app (sum, average, min, max, etc). To add the average formula to the right-most column in our spreadsheet, all we need to do is drag and drop the pre-populated calculation from the lower left-hand corner to the cell where we want to place the formula. That's it!
Here's a video to visually walk you through the process: