Question

A motor in still water travels at a speed of 36 km/hr. It goes 56 km upstream in 1 hour 45 min. The time taken by it to cover the same distance downstream is 

(a) 2 hours 25 min 

(b) 3 hours 

(c) 1 hour 24 min 

(d) 2 hours 21 min

Answer 1

(c) 1 hour 24 min

Let the speed of the stream be x km/hr

Then, \(\frac{56}{\frac74}=36-x\) ⇒ \(\frac{56\times4}{7}=36-x\)

⇒ x = 36 – 32 = 4 km/hr 

∴ Time taken by it to cover the same distance 

upstream = \(\frac{56}{36+4}\) hrs = \(\frac{56}{40}\)

= \(1\frac25\) hours = 1 hour 24 min.