A boat travels with a speed of 15 km/hr in still water. In a river flo

1.	A boat travels with a speed of 15 km/hr in still water. In a river flowing at 5 km/hr, the boattravels some distance downstream and then returns. The ratio of average speed to the speedin still water isrio(A) 8:3(B) 3:8(C) 8:9(D) 9:8
Answer» Let the distance travelled be x km speed downstream = 15+ 5 = 20km/h speed upstream = 15-5 = 10 km/h Total distance = 2x Total time = x/20 + x/10 = 3x/20 Average speed = Total distance/Total time = 2x / (3x/20) = 40/3 The ratio of average speed to the speed in still water = 40/3 : 15 = 40 : 45 = 8 : 9

A boat travels with a speed of 15 km/hr in still water. In a river flowing at 5 km/hr, the boattravels some distance downstream and then returns. The ratio of average speed to the speedin still water isrio(A) 8:3(B) 3:8(C) 8:9(D) 9:8

Answer»

Let the distance travelled be x km

speed downstream = 15+ 5 = 20km/h

speed upstream = 15-5 = 10 km/h

Total distance = 2x

Total time = x/20 + x/10 = 3x/20

Average speed = Total distance/Total time

= 2x / (3x/20)

= 40/3

The ratio of average speed to the speed in still water = 40/3 : 15

= 40 : 45

= 8 : 9