Multiple choice

The average age of a class is 12.05 years. The average age of all girls is 12.5 years and average age of all boys is 11.75 years. If total number of boys is 45, what is the total number of girls?

  1. 20

  2. 25

  3. 30

  4. 35

  5. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Using weighted average formula: Total avg = (Boys × Boy avg + Girls × Girl avg) / (Boys + Girls). Let g = number of girls. 12.05 = (45 × 11.75 + g × 12.5) / (45 + g). Solving: 12.05(45 + g) = 528.75 + 12.5g, 542.25 + 12.05g = 528.75 + 12.5g, 13.5 = 0.45g, g = 30.

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