Question

Fill in the blank:

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

Answer 1

Answer: (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 $=\frac{3x+5x+7x}{3}$
$25=\frac{15x}{3}$
$25=5x$
$x=5$
The age of the youngest boy $=3x=3\times 5=15$ years