A motorboat’s speed in still water is 45 km/h. It covers 80 km with the stream in 1 hour 20 minutes. Find the time it will take to cover the same 80 km against the stream.

Introduction / Context:
From the downstream leg we can infer the current and then compute the upstream speed and time for the same distance.

Given Data / Assumptions:

• b = 45 km/h
• Downstream distance = 80 km in 1 h 20 min = 4/3 h

Concept / Approach:
Downstream speed vd = distance/time = 80 / (4/3) = 60 km/h ⇒ current c = vd − b. Then upstream speed vu = b − c and time = 80 / vu.

Step-by-Step Solution:

Verification / Alternative check:
Check dimensional consistency and reasonableness: upstream slower than b; time larger than 1 h 20 min, as expected.

Why Other Options Are Wrong:
Any time other than 2 h 40 min conflicts with the computed upstream speed 30 km/h over 80 km.

Common Pitfalls:
Using 45 km/h directly for upstream or not converting 1 h 20 min to hours correctly.

Final Answer:
2 h 40 min.