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Five years ago the average of the ages of A and B was 40 years and now the average of the ages of B and C is 48 years. What will be the age of the B ten years hence?

Correct answer: D. Data inadequate

  • A. 55 years
  • B. 56 years
  • C. 58 years
  • D. Data inadequate

Explanation

Five years ago, A and B together were 2 × 40 = 80 years old, so now their combined age is 90 years. Now B and C together are 2 × 48 = 96 years old, but these two equations do not determine B's individual age. The likely mistake is choosing 55 years by treating B as half of the first or second pair's total. “Data inadequate” is therefore correct.

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About Averages

An average represents the total of a set of values divided by the number of values, and a weighted average gives different importance to different groups. Problems involve missing values, combined averages, replacement of observations, and changes in the average, so the total and count must be tracked separately.

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