This answer choice is formed by multiplying by 5 for every new question. We can think about it like this:
For question 1, we have 5 choices - let's say A, B, C, D, and E. So we have 5 possible answer keys so far.
For question 2, we have 5 more choices. So if A is in the answer key for question 1, we can have A, B, C, D, or E for question 2. That's 5 options. If B is in the answer key for question 2, we can again have A, B, C, D, or E for question 3. That's 5 more options. The same is true for C, D, and E for question 1 - 5 more options for each. So for each possible answer key that we had before question 2, we got 5 more options. We have 5 possible answer keys, so know we have 5 * 5 = 25.
For question 3, we have 5 more choices. We already have 25 possible answer keys, so we get 5 more options for each of those: 25 * 5 = 125.
So each question multiplies our number of possible answer keys by 5. We have 60 questions, so we have 5 * 5 * 5 ... 60 times, or 5^60.
This principle is also known as
the rule of product, or the multiplication principle. It's pretty commonly tested, so being comfortable with using it will help you out a lot on test day.
