The average age of three boys is $25$ years and their ages are in the proportion $3:5:7$. The age of the youngest boy is ________.

A

$21$ years

B

$18$ years

C

$15$ years

D

$9$ years

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Solution

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Correct option is

C

$15$ years

average age of three boys $=25$ years, their ages are in the proportion $3:5:7$ Lets us assume their ages $=3x,5x,7x$ As we know, an average is the sum of a list of numbers divided by the number of numbers in the list. Average age of three boys $=33x+5x+7x $ $25=315x $ $25=5x$ $x=5$ The age of the youngest boy $=3x=3×5=15$ years