Question

A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will he take to go 5 km in stationary water? 

(a) 1 hour 

(b) \(1\frac12\) hours 

(c) 1 hour 15 min 

(d) 40 min

Answer 1

(c) 1 hr. 15 min

Speed of the boatman upstream = \(\frac21\) km/hr = 2 km/hr 

Speed of the boatman downstream = \(\frac{1}{\frac{10}{60}}\) km/hr = 6 km/hr

∴ Speed of boatman in stationary water = \(\frac12(6+2)\) km/hr = 4 km/hr

∴ Time taken to cover 5 km in stationary water = \(\frac54\) hrs = \(1\frac14\) hrs = 1 hr. 15 min.