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 Mean: Find the missing number To find a missing number when the average (mean) of all the numbers in a set is given, plug into the arithmetic-mean formula all the numbers you know -which include the average, the sum of the other numbers, and the number of terms. Then, use algebra to find the missing number. The average of five numbers is 26. Four of the numbers are -12, 90, -26, and 10. What is the fifth number? To solve the problem algebraically, let x=the missing number. Set up the artithmetic-mean formula, then solve for x: $26=\frac{(90+10-12-26)+x}{5}$ $26=\frac{62+x}{5}$ 130 = 62+x 68 = x The fifth number is 68.