A man rows 3⁄4 km upstream in 11¼ minutes and the same distance downstream in 7½ minutes. What is his speed in still water (km/h)?

Given data

• Distance each way = 0.75 km
• Upstream time = 11.25 min = 11.25/60 h
• Downstream time = 7.5 min = 7.5/60 h

Concept / Approach

• Compute upstream speed (u) and downstream speed (d), then still-water speed = (u + d)/2.

Step-by-step calculationu = 0.75 ÷ (11.25/60) = 0.75 ÷ 0.1875 = 4 km/hd = 0.75 ÷ (7.5/60) = 0.75 ÷ 0.125 = 6 km/hStill-water speed = (4 + 6)/2 = 5 km/h

VerificationCurrent speed = (d − u)/2 = (6 − 4)/2 = 1 km/h; times match given data.

Final Answer5 km/h