Answer is: (b) 15 years.
Total age of 30 boys = 30 × 15 = 450
Total age of the rest of boys after the boy 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
⇒ 435 + 2x = 31 × 15 = 465
⇒ 2x = 30 ⇒ x = 15 years.