Question

A motor boat takes 6 hours to cover 100 km downstream and 30 km upstream. If the motor boat goes 75 km downstream and returns back
to its starting point in 8 hours, find the speed of the motor boat in still
water and the speed of the stream.

Answer 1

Given :

A motor boat takes 6 hours to cover 100 km downstream and 30 km upstream. If the motor boat goes 75 km downstream and returns back to its starting point in 8 hours

To find :

find the speed of the motor boat in still

water and the speed of the stream.

Solution :

let speed of motor boat in still water = x km/hr

speed of stream = y km/hr

speed of motor boat in down stream = (x + y) km/hr

speed of motor boat in upstream = (x - y) km/hr

100/(x + y) + 30/(x - y) = 6 ....... (1)

75/(x + y) + (75/x - y) = 8 ........(2)

solving 1 and 2 we get

x + y = 25 , x - y = 15

x = 20 , y = 5

=> speed of motor boat in still water = 20 km/hr , speed of stream = 5 km/hr