In a school, there are 40 students in which boys and girls are in the ratio 3:1. The mean age of the boys is 20 years and the mean age of girls is 18 years. If there is increase in girls by 5 and decrease in boys by 10, then find the new average age (approx), when the students joining or leaving doesn't change the average age of boys and girls respectively.

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Solution

$40$ student = boys $= \frac{3}{4} \times 40 = 30$
Girls $= 1 / 4 \times 40 = 10$
< age boys> $= 20 \times 30 = 600$
< age girl > $= 18 \times 10 = 180$
$\Rightarrow$ girls $= 15$ boys $= 20$
< boys> $= 600 / 20 = 30$
< girl > $= 180 / 15 = 12$

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