The ratio of the speed of a boat in still water to the speed of the stream is 5:1. The boat covers 72 km downstream and 48 km upstream in a total of 8 hours. What is the total time taken by the boat to cover 120 km downstream and 90 km upstream if the speed of the stream is increased by 2 km/h?

Let the speed of the boat in still water be 5x km/h and the speed of the stream be x km/h. So, downstream speed = 5x + x = 6x km/h And, upstream speed = 5x – x = 4x km/h ATQ, 72/6x + 48/4x = 8 Or, 12/x + 12/x = 8 Or, 24/x = 8 So, x = 3 Speed of the boat in still water = 5 × 3 = 15 km/h Speed of the stream = 3 km/h New speed of the stream = 3 + 2 = 5 km/h New downstream speed = 15 + 5 = 20 km/h New upstream speed = 15 – 5 = 10 km/h So, total time taken = 120/20 + 90/10 = 6 + 9 = 15 hours Hence, the correct answer is C.