In a class, the average age of girls is 21 yearsand the average age of boys is 4 years less than the average age of the entire class. Find the average age of the entire class, if the ratio of the number boys and girls is 3:4.

20 years

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21 years

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19 years

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18 years

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Solution

The correct option is D 18 years
Let 3x and 4x be the number of boys and girls in the class and the average age of the entire class be a years, then

7 × a = 3× (a – 4) + 4 × 21

⇒ 7a = 3a – 12 + 84

⇒ 4a = 72

⇒ a = 18

Hence, option D is the correct answer.

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