Question

A man rows a boat 18 km in 4 hours downstream and returns upstream in 12 hours. The speed of the stream (in km/hr) is 

(a) 1 

(b) 1.5 

(c) 2 

(d) 1.75

Answer 1

(b) 1.5 km/hr

Let the speed of the boat in still water = x km/hr and the speed of the stream y km/hr. 

Then, speed downstream = x + y = \(\frac{18}{4}=\frac92\)      ...(i)

and speed upstream = x – y = \(\frac{18}{12}=\frac32\)           ....(ii)

Subtracting eq (ii) from eq (i), we get,

2y = \(\frac62\) = 3 ⇒ y = 1.5 km/hr.