The speed of the motor boat in still water is 25/3 m/s. It goes 75 km............Find the speed of the current in km/hr.

Question

The speed of the motor boat in still water is 25/3 m/s.
It goes 75 km downstream and comes back in 9 hours.
Find the speed of the current in km/hr.

Answer 1

(25/3) *(3600/1000) = 30 Km/hr
downstream time ==> (75/(30 + x))
upstream time ==> (75/(30 - x))
(upstream + downstream) time = 9 hours.
75*(1/(30 + x) + 1/(30 - x)) = 9
75*(30 - x + 30 + x)/(900 - x^2) = 9
75*60/9 = 900 - x^2
x^2 = 900 - 500 = 400
==> x = 20 Km/Hr. = Speed of the current.