A boatman goes 2 Km against the current of the stream in 2 hour and goes 1 km along the current in 20 minutes. How long will it take to go 5 km in stationary water?

Speed of boatman=(Upstream + Downstream)/2
Speed=Distance / Time
Speed of upstream against water=2km per 2 hr=2/2=1Km/hr
Speed of downstream=1 km per 20 min=1*60/20=3km/hr
Total Speed of boatman=1+3/2=4/2=2 km/hr
Time taken for 5 km in stationary water=5/2=2.5h
1.5hr=2 hour 30 min