I finally found this in Excel Help - couldn't find this earlier:
Excel provides different ways to calculate percentages. For example, you can use Excel to calculate sales tax for purchases, the percentage of correct answers on a test, or the percentage of change between two values.
Use the provided sample data and the following procedures to learn how to calculate percentages.
Calculate the percentage if you know the total and amount
Let's say that you answered 42 questions out of 50 correctly on a test. What is the percentage of correct answers?
1. Click any blank cell.
2. Type =42/50, and then press RETURN.
The result is 0.84.
3. Select the cell that contains the result from step 2.
4. On the Home tab, under Number, click Percentage .
The result is 84.00%, which is the percentage of correct answers on the test.
Note To change the number of decimal places that appear in the result,
click Increase Decimal or Decrease Decimal .

=K37/$F$38 -→73% got this item correct in the test (and item analysis)
K37 is the number of correct answers in this class, for this question; $F#38 is the total number of students in that class