M4maths Previous Puzzles

21 Jul 2020 Boats and Streams

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.

OPtion

1) 50/9
2) 20
3) 13
4) 13.33
5) 20
6) 15
7) 18
8) 14
9) 12
10) None of these

Solution

Let the speed of the current = 'x' km/hr

Speed of motor boat = 25/3 m/s = (25/3)*18/5 = 30 KM/hr

We have 75/(30+x) + 75/(30-x) = 9
⇒ 75(60/(900-x2)) = 9
⇒ 900-x2 = 500
⇒ x2 = 400
⇒ x = 20km/hr

Correct Option: 5 Best Solution (0)