To solve this problem, let's represent the boat's speed in still water by "x" km/h.
When the boat is traveling downstream (along the current), its effective speed will be the sum of the boat's speed in still water and the speed of the current. Thus, the boat's speed downstream will be (x + 1.5) km/h.
When the boat is traveling upstream (against the current), its effective speed will be the difference between the boat's speed in still water and the speed of the current. Thus, the boat's speed upstream will be (x - 1.5) km/h.
Given that the boat can travel twice the distance downstream than it can upstream in one hour, we can set up the equation:
Distance downstream = 2 * (Distance upstream)
Since speed is equal to distance divided by time, and the time is the same for both downstream and upstream, we can set up the following equation:
(x + 1.5) = 2 * (x - 1.5)
Now, let's solve for x:
x + 1.5 = 2x - 3
1.5 + 3 = 2x - x
4.5 = x
Therefore, the boat's speed in still water is 4.5 km/h.