7 pupils of average age 12 years leave a class of 25 pupils of average age 14 years. If 6 new pupils of average age 11years join the class, what is the average age of the pupils now in the class?
 

  • A 13years
  • B 12 years 7\(\frac{1}{2}\) months
  • C 13 years 5 months
  • D 13 years 10 months
  • E 11 years
jamb 1981mathematics

The correct answer is D. 13 years 10 months

The total age of the 25 pupils before the 7 pupils left was 25 * 14 = 350 years. 

After the 7 pupils of average age 12 years left, the total age of the remaining pupils was 350 - 7 * 12 = 266 years. 

When the 6 new pupils of average age 11 years joined the class, the total age of the pupils in the class became 266 + 6 * 11 = 332 years. 

The number of pupils in the class is now 25 - 7 + 6 = 24. 

So, the average age of the pupils now in the class is 332 / 24 = 13 years and \(\frac{10}{12}\) months, or 13 years and \(10\) months.

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