Question 64081
Let x=boat's speed in still water and
Let y=speed of current

The boat's speed against the current (x-y) was 18km/3hr=6km/hr.
The boat's speed with the current (x+y) was 18km/1.5hr=12km/hr.
so we have:

(1)  x-y=6km/hr
(2)  x+y=12km/hr

adding the equations, we get:
2x=18km/hr divide both sides by 2 we have:
x=9km/hr speed of the boat in still water

substitute in (1) and we get:

9-y=6
-y=6-9
y=3km/hr speed of the current.

Hope this helps.  Have a nice holiday season.----ptaylor