Explanation:
prime numbers from 1 to 20 are those that can be divided by 1 and itself, so
1
2
3
5
7
11
13
17
19
the average is the sum of all of them, divided by how many they are (there are 9 of them)
avg = (1+2+3+5+7+11+13+17+19)/9
avg = 8.67
note: some consider that 1 is not a prime number because in this case the definition "that can be divided by 1 and itself" is redundant. if that's what you learned in class, then you need to remove it from the formula and divide by 8