A steamer goes downstream from one port to another in 4 hours, it covers the same distance upstream in 5 hours. If the speed of the stream is 2km/hr,the distance between the two parts is_______?

A. 50km
B. 60km
C. 70km
D. 80 km
Explanation:
Let the distance between the two parts be x km.
Then speed downstream = x/4 km/hr.
Speed Upstream = x/5 km/hr
Speed of the stream = ½ (x/4 –x/5)
Therefore, ½(x/4 –x/5) = 2. => x/4 –x/5 = 4 => x = 80.
Hence, the distance between the two ports is 80km.