Question

The average age of 30 boys in class is 15 years. One boy aged 20 years left the class, but two new boys come in his place whose ages difference by 5 years. If the average age of all the boys now in the class still remains 15 years, the age of the younger new comer is

• A. 20 years
• B. 15 years
• C. 10 years
• D. 8 years

Answer 1

Answer: (B)

15 years

Total age of 30 boys = 30 x 15 = 450
Total age of the rest of boys after the bot aged 20 years leaves = 450 - 20 = 430
Let x be the age of the younger new comer.
Then, $\frac{430+x+(x+5)}{31}=15$
$\Rightarrow 435+2x=31\times 15=465$
$\Rightarrow 2x=30\Rightarrow x=15$ years