The current of a stream runs at 1 km/hr. A motor boat goes

Home » Aptitude » Boats and Streams » Question

The current of a stream runs at 1 km/hr. A motor boat goes 35 km upstream and back again to the starting point in 12 hours. The speed of motor boat in still water is ?

1. 6 km/hr
2. 7 km/hr
3. 8.5 km.hr
4. 8 km/hr

Correct Option: A

Let the speed in still water be x km/hr
∵ 35/(x - 1) + 35/(x + 1) = 12
⇒ 35(2x) = 12(x² - 1)
⇒ 12x² - 70x - 12 = 0
⇒ 12x² - 72x + 2x - 12 = 0
⇒ 12x(x - 6) + 2 (x - 6) = 0
⇒ (x - 6) (12x + 2) = 0
∴ x = 6 km/hr