A man can row 5 km/h in still water. If the speed of the current is 2.5 km/h, it takes 4 hours more in upstream than in the downstream for the same distance. The distance is:

15 km

We know that,

Distance = Speed × Time

Given,

Speed in still water = 5km/h

Speed of upstream = 5km/h - Current speed = 5 - 2.5 = 2.5km/h

Time in upstream = 4 h + Time in Downstream

According to the question,

Distance for Upstream = Distance for downstream

= 2.5 (4 + T) = 7.5T

= 10 + 2.5T = 7.5T 

= 5T = 10

= T = 2 h

Now substituting the value of T in Downstream then

= D = 7.5 × 2 = 15km