Total age of 30 boys = 30 x 17 = 510 If one boy aged 18 years leaves, then total age will be (510 - 18) = 492 Total age after the new boy joins the class = 16.9 x 30 = 507 ∴ Age of the new boy = 507 - 492 = 15 years Hence, option B is the correct answer.