What is the average of these numbers? : 5, 10, 15 and 20?

To calculate the average among a group of numbers, we can add up all the numbers first, then divide the total by the number of the numbers in the group.

So in this question, first add up all the numbers.
5 + 10 + 15 + 20
=50

Now divide 50 by 4 since we have 4 numbers in the group.
50 / 4
= 12. 5

Therefore, your answer is 12.5.