, ,

The mean of a set of numbers is the sum divided by the number of terms.

Factor out of .

Factor out of .

Factor out of .

Factor out of .

Factor out of .

Cancel the common factors.

Factor out of .

Cancel the common factor.

Rewrite the expression.

Divide by .

Add and .

Add and .

Find the Mean (Arithmetic) 3 , 12 , 21