Multiple choice

A group of boys has an average weight of 36 kg. One boy weighing 42 kg leaves the group and another boy weighing 30 kg joins the group. If the average now becomes 35.7 kg, then how many boys are there in the group?

  1. 30

  2. 32

  3. 40

  4. 56

  5. 60
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let n be the number of boys. Total weight = 36n. New total weight = 36n - 42 + 30 = 36n - 12. New average = (36n - 12) / n = 35.7. 36n - 12 = 35.7n. 0.3n = 12. n = 12 / 0.3 = 40.

AI explanation

Let the number of boys be n. The total weight decreases from 36n to 35.7n when a 42 kg boy is replaced by a 30 kg boy. The net decrease in total weight is 12 kg, so 36n - 35.7n = 12. Solving 0.3n = 12 gives n = 40.

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