The ratio of the speed of boat A in still water to that of boat B in still water is 5:6. Both boats are in a river where the speed of the stream is 3 km/h. If boat A can cover 126 km downstream in 7 hours, find the time taken by boat B to cover 120 km upstream?

ATQ, Downstream speed of boat A = 126 / 7 = 18 km/h Speed of boat A in still water + Speed of stream = 18 Or, Speed of boat A in still water + 3 = 18 So, Speed of boat A in still water = 15 km/h Ratio of speed of boat A to boat B in still water = 5:6 Here, 5 parts represent the speed of boat A in still water. So, 5 parts = 15 km/h Or, 1 part = 3 km/h Speed of boat B in still water = 6 parts = 6 × 3 = 18 km/h Upstream speed of boat B = Speed of boat B in still water - Speed of stream Or, Upstream speed of boat B = 18 - 3 = 15 km/h Time taken by boat B to cover 120 km upstream = 120 / 15 = 8 hours Hence, the correct answer is B.