the average of a set of numbers is the quotient obtained by dividing the sum of the members of the set by the?

Answer 1

...number of members in the set.

For example, in the following set:
{10, 20, 30, 40, 50, 60, 70, 80, 90, 100}

The sum is:
10+20+30+ ... + 90+100 = 550

The number of members in the set is:
10

The average is given by the quotient:
= 550 / 10
= 55

So the answer is the "...number of members in the set"

Answer 2

Number of the given members.

Answer 3

basically the answers above, but the we do use the word 'cardinality' to mean number of elements. So, you could complete your statement with "by the cardinality of the set."