Question

the motor boat goes downstream and cover a distance in 4 hours while it cover the same distance upstream in 5 hour if the speed of the stream is 3 kilometre per hour find the speed of motor boat in still water

Answer 1

let the speed of boat be x km.
in the case of downstream the speed of stream is added to speed. therefore speed in downstream in 1 hour = x+3 km
speed in 4 hour = 4(x+3) km

in the case of upstream the speed of stream is subtracted to speed. therefore the speed in 1 hour = x-3km
speed in 5 hour = 5(x-3)km

now according to question, both covers the same distance

4(x+3)=5(x-3)
4x+12=5x-15
4x-5x=-15-12
-x = -27
x=27km/hr