Trains Boats and Streams Practice Test 3 with solutions

Question : 4 of 40

Marks: +1, -0

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

9 km/hr
8 km/hr
7 km/hr
6 km/hr

Validate

Solution:

Let x km/hr be the speed of the motorboat in still water, then,

{35}/{x-1}+{35}/{x+1}

= 12
⇒

{35x}/{x^2-1}

= 6
⇒

6x^2

- 35x - 6 = 0
⇒ (6x + 1) (x – 6) = 0
Hence, x = 6 km/hr.

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