A boat can cover 72 km upstream and 180 km downstream in 12 hours. Speed of the stream is how much less than the speed of the boat in still water if the boat can cover 54 km upstream and 150 km downstream in 9 hours?

ATQ, ATQ, Let the upstream speed and downstream speed of the boat be x km/h and y km/h, respectively. So according to question: 72/x + 180/y = 12 ………………. (i) Also, 54/x + 150/y = 9 ……….. (ii) Solving (i) and (ii), we get x = 15 and y = 30 So, the upstream speed and downstream speed of the boat are 15 km/h and 30 km/h, respectively. Speed of the boat in still water = (15 + 30)/2 = 22.5 km/h Speed of the stream = (30 – 15)/2 = 7.5 km/h So, the desired difference = 22.5 – 7.5 = 15 km/h