Question

A motor boat whose speed is 9km/hr in still water goes 15km downstream and comes back in upstream in a total time of 3 hours and 45 minutes.Find the speed of the stream

Answer 1

solution

T = time for downstream,

t = time for upstream travel.

T + t = 3 hrs 45 min = 15/4 hrs.

For downstream, distance/velocity = time,

15/(V + v) = T

15/(9 + v) = T ... (1)

Similarly,

15/(9 - v) = t ... (2) ...where V - Speed of boat, v - speed of stream.

Add (1) and (2)

15/(9 + v) + 15/(9 - v) = T + t

15(18)/(81 - v2) = 15/4

v2 = 81 - (18x4) = 9

v = sped of stream is 3 km/hr.