A boat covers a certain distance downstream in 1 hour, while it comes back in 1.5 hour. If the speed of the stream be 3 km/hr, what is the speed of the boat in still water?

A 12 km/hr

B 9.5 km/hr

C 14 km/hr

D 7.5 km/hr

E None of these

Correct Answer: Option E

Let the speed of the boat in still water be x km/hr

Speed downstream = (x + 3) km/hr

Speed upstream = (x − 3) km/hr

Distance of downstream = Distance of Upstream

∴ (x + 3) × 1 = (x − 3) × 1.5

⇒ x + 3 = 1.5x – 4.5

⇒ 0.5x = 7.5 km/hr

⇒ x = 15 km/hr