Current speed from downstream and upstream rates: Keshav can row 60 km downstream in 10 h and 36 km upstream in 10 h. What is the speed of the current?

Introduction / Context:
Given distances and times for downstream and upstream trips, we compute effective speeds for each direction and then use their difference to find the current speed. This is a core pattern in boats and streams problems.

Given Data / Assumptions:

• Downstream: 60 km in 10 h
• Upstream: 36 km in 10 h
• Uniform current, straight river

Concept / Approach:
sd = distance/time downstream; su = distance/time upstream. Then still water u = (sd + su)/2 and current v = (sd - su)/2. We only need v.

Step-by-Step Solution:

Verification / Alternative check:
Still water u = (6 + 3.6)/2 = 4.8 km/h. Check: u + v = 6 and u - v = 3.6 as required.

Why Other Options Are Wrong:

• 2.2 km/h and 4.0 km/h do not match (sd - su)/2.
• 3.0 km/h is the half-sum mistake or random guess.
• 0.8 km/h is too small to account for the observed difference.

Common Pitfalls:
Taking average of distances or times instead of effective speeds; mixing upstream and downstream speeds. Always form sd and su first, then compute v.

Final Answer:
1.2 km/h