Multiple choice

The average age of four boys, five years ago, was 9 years. On including a new boy, the present average age of all the five is 15 years. The present age of the new boy is

  1. 14 years

  2. 6 years

  3. 15 years

  4. 19 years
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D Correct answer
Explanation

Five years ago, the sum of ages of 4 boys was 9 * 4 = 36. Today, their sum is 36 + (5 * 4) = 56. The new average age of 5 boys is 15, so their total age is 15 * 5 = 75. The new boy's age is 75 - 56 = 19.

AI explanation

Five years ago, the average age of the four boys was 9 years, making their total age 4 multiplied by 9, which is 36 years. In the present day, the total age of these four boys is 36 plus (4 multiplied by 5), equaling 56 years. The present average age of all five boys is 15 years, so their total present age is 5 multiplied by 15, which equals 75 years. The present age of the new boy is the difference between the total present age of all five boys and the present age of the four original boys, which is 75 minus 56, giving 19 years.

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