Math ?
you must answer exactly 12 out of 15 questions on a test. how many different ways can you select the questions to answer?

Basically you want to calculate how many ways you can choose 12 objects from a set of 15.
The formula is n! / (r! * (n-r)!)
15! / 12! * (15 -12)! =
15*14*13 * 12! / 12! * 3!  both 12! terms cancel out =
15*14*13 / 3 * 2 =
5  * 7 * 13 =
455