Explanation : Let present age grow son's age = x yr Then, 6x + 4 = 4(x + 4). Then present age of father = 6x yr ⇒ 2x = 12 ∴ x = 6 yr ∴ Son's age = 6yr and Father's age = 6 x 6 = 36 yr.
Explanation : Throwing a dice can result in any one of the six outcomes 1,2, ....6. Prime numbers from 1 to 6 are 2,3,5. ∴ Required probability = 3/6 = 1/2.