The tip
Any time an average problem has a missing value, multiply the mean by the count to get the total. The missing number is just the total minus what you know. The enhanced ACT math section gives you about a minute per question, so this shortcut matters.
Try it
The average of five numbers is 18. Four of the numbers are 12, 15, 20, and 25. What is the fifth number?
- A16
- B18
- C20
- D22
Reveal answer
Answer: B. 18
The total is 5 x 18 = 90. The four known numbers add to 12 + 15 + 20 + 25 = 72. The fifth number is 90 - 72 = 18.
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