Question

Two ghats are located on a riverbank and are 21 km apart. Leaving one of the ghats for the other, a motorboat returns to the first ghat in 270 minutes, spending 40 min of that time in taking the passengers at the second ghat. Find the speed of the boat in still water if the speed of the river flow is 2.5 km/h.

• A. 10.4 km/h
• B. 12.5 km/h
• C. 22.5 km/h
• D. 11.5 km/h

Answer 1

The correct option is D 11.5 km/h

Let us consider the speed of the boat in still water as 'b' km/hr.
$\frac{21}{(b+2.5)}+\frac{21}{(b-2.5)}=\frac{230}{60}$
$23\times b^2-252\times b-143.75=0$
b = 11.5 km/hr