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ASVAB · Averages and Statistics

The average of five numbers is 20. If four of the numbers are 18, 22, 25, and 15, what is the fifth number?

Correct answer

20

  1. A 20
  2. B 18
  3. C 20.5
  4. D 100

Why this is the answer

If the average of five numbers is 20, their total is 5 × 20 = 100. The four known numbers add to 18 + 22 + 25 + 15 = 80. The fifth number is 100 − 80 = 20. The method for a missing value in an average is always the same: multiply the average by the count to get the required total, then subtract the known values. The fifth number is 20. The distractor 100 is the total of all five numbers, not the missing one, so be sure to finish the subtraction.
Source: ASVAB AR, Missing Value in Average

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