A man takes 4 hours and 30 minutes to row a boat a distance of 18 km downtream, and 3 hours 30 minutes to row the boat a distance of 7 km upstream. Find the speed of the stream.

Let's assume the speed of boat in still water and the speed of stream are 'B' and 'C'.

A man takes 4 hours and 30 minutes to row a boat a distance of 18 km downtream.

$$relative\ speed=\frac{distance}{time}$$

$$B+C\ =\ \frac{18}{4.5}$$

B+C = 4    Eq.(i)

3 hours 30 minutes to row the boat a distance of 7 km upstream.

$$relative\ speed=\frac{distance}{time}$$

$$B-C\ =\ \frac{7}{3.5}$$

B-C = 2    Eq.(ii)

Add Eq.(i) and Eq.(ii).

B+C+B-C = 4+2

2B = 6

B = 3 km/h

Put the value of 'B' in Eq.(i).

3+C = 4

C = 4-3

Speed of the stream = C = 1  km/h