A boat covers a distance downstream in 1 hour and upstream in 1½ hours. If the stream's speed is 3 km/h, what is the boat's speed in still water?

Given data

• Let the one-way distance be D km.
• Downstream time = 1 h ⇒ D = (v + 3) × 1
• Upstream time = 1.5 h ⇒ D = (v − 3) × 1.5

Concept / Approach

• Equate the two expressions for D and solve for still-water speed v.

Step-by-step calculation
v + 3 = 1.5(v − 3)v + 3 = 1.5v − 4.5 ⇒ 7.5 = 0.5v ⇒ v = 15 km/h

Verification
Downstream speed 18 ⇒ D = 18 km; Upstream speed 12 ⇒ time = 18/12 = 1.5 h ✔

Final Answer
15 km/h