A man rows 3/4 km (0.75 km) against the stream in 11 minutes 15 seconds and returns the same 0.75 km downstream in 7 minutes 30 seconds. Find his speed in still water (in km/h).

Introduction / Context:When distances are equal, we can compute separate effective speeds from time and then recover the still-water speed as the average of downstream and upstream speeds.

Given Data / Assumptions:

• Distance each way = 0.75 km
• Upstream time = 11 min 15 s = 11.25 min = 0.1875 h
• Downstream time = 7 min 30 s = 7.5 min = 0.125 h

Concept / Approach:Compute vu and vd from distance/time. Then still-water speed b = (vd + vu)/2.

Step-by-Step Solution:

Verification / Alternative check:Current c = (vd − vu)/2 = (6 − 4)/2 = 1 km/h, so b − c = 4 and b + c = 6 as required.

Why Other Options Are Wrong:2, 3, 4 km/h do not average the computed leg speeds and would not reproduce both 11.25 min and 7.5 min times for 0.75 km.

Common Pitfalls:Confusing still-water speed with downstream speed; ensure to average vd and vu, not the times.

Final Answer:5 km/hr.