A man can row 4.5 km/hr in still water and he finds that it takes him twice as long to row up as to row down the river. Find the rate of the stream ?

Let the speed of the current be x km/hr.
Upward speed = (4.5 + x) km/hr
Downward speed = (4.5 − x) km/hr.
Let distance travelled be y, then
y / (4.5 − x) = 2y / (4.5 + x)
x = 1.5 km/hr