Multiple choice

The average of the first four numbers is three times the fifth number. If the average of all the five numbers is 85.8, then the fifth number is:

  1. 33

  2. 34

  3. 39

  4. 29
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A Correct answer
Explanation

Let the first four numbers' average = A, so their sum = 4A. Given A = 3 × fifth number (f). So sum of first four = 4(3f) = 12f. Average of all five = (12f + f)/5 = 85.8, so 13f/5 = 85.8, thus 13f = 429, f = 429/13 = 33. The fifth number is 33. The key is setting up the relationship between averages correctly.

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