A motorboat in still water travels at a speed of 36 km/h. It goes 56 km upstream in 1 hour 45 minutes. The time taken by it to cover the same distance down the stream will be:

A 2 hours 25 minutes

B 3 hours

C 1 hour 24 minutes

D 2 hours 21 minutes

E 1 hour 40 minutes

Correct Answer: Option C

Given: speed of motor boat in still water = 36 km/hr

Let the time taken to cover same distance down the stream = x hr

Speed of boat upstream = \(\frac{{56\;km}}{{1\;hr\;45\;min}}\; = \frac{{56}}{{\frac{7}{4}}} = 32{\rm{\;km}}/{\rm{hr}}\)

Man’s rate in still water = (rate in upstream + rate in downstream)/2

\(36 = \frac{32+\frac{{56}}{x}}{2}\)

⇒ x = (56/40)

⇒ x = (14/10) hrs

\(= 1\frac{4}{{10}}\;hrs\)

= 1 hr 24 mins