A man can row up stream at 4 km/h and downstream at 7 km/h. What is the rate of the current?

1.5 km/h

Let,

Speed of boat in still water= x

Speed of stream/current = y

According to the question:

Upstream speed (Us) = 4 km/hr i.e. x - y = 4km/hr -----1st equation

Downstream speed (Ds) = 7km/hr i.e. x + y = 7km/hr -----2nd equation

Subtracting 1st eq from 2nd we get 

x + y - ( x- y) = 7 - 4

x + y - x + y = 3

2y = 3

y = \(\frac{ 3}{ 2}\) = 1.5 km/hr