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The average age of three boys is 15 yr. If the ratio of their ages is 3 : 5 : 7, What is the age of the oldest boy?

7 yr
14 yr
20 yr
21 yr

Correct Answer: 21 yr

Explanation:

Total age of all three boys
= 3 x 15 = 45 yr
Ratio of ages = 3 : 5 : 7
∴ Age of the oldest boy
= [7/(3 + 5 + 7)] x 45 = (7/15) x 45 = 21 yr

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