The ratio of the ages of A and B is 4 : 5. After 6 years, the ratio becomes 6 : 7. What is the present age of B?

माना आयु 4x और 5x है। (4x+6)/(5x+6) = 6/7 -> 28x + 42 = 30x + 36 -> 2x = 6 -> x = 3। B की आयु = 5 * 3 = 15 वर्ष।