The mean of a data set is found by adding all the members of the data set, then dividing by the total number of members.
{ 2, 3, 4, 5, 6}
2 + 3 + 4 + 5 + 6 = 20
20 ÷ 5 = 4
mean = 4

You can use algebraic equations to discover missing numbers from the data set and how many members are in the data set.
Let's look at finding a missing member.

{17, n , 47} ; mean = 32
So (17 + n + 47) ÷ 3 = 32.

Solve for n
{18, n , 25} ; mean = 21

Solve for n
{9, 12, 17, 21, 22, n} ; mean = 15

Carol has the following 4 test scores. What does she need on the 5^th test to have an A average?
{89, 87, 96, 98, n} ; mean = 93

With one game to go, William has already bowled a 96 and a 107. His team needs him to average 105 to win. What must he bowl in the third game for his team to win?
{96, 107 n} ; mean = 105