Jane took $30 \mathrm{~min}$ to drive her boat upstream to water-ski at her favorite spot. Coming back later in the day, at the same boat speed, took her $20 \mathrm{~min}$. If the current in that part of the river is $7 \mathrm{~km}$ per hr, what was her boat speed in still water?

Step 1 ：Let's denote the boat speed in still water as x (km/hr). The distance she travelled upstream and downstream is the same. We can set up two equations based on the given information and solve for the boat speed.

Step 2 ：Time = Distance/Speed. For upstream, we have 30 min = Distance/(x-7).

Step 3 ：For downstream, we have 20 min = Distance/(x+7).

Step 4 ：The solution to the system of equations is not as expected. It seems there was a mistake in the formulation of the equations. The distance should be the same for both upstream and downstream, so we should set the two distances equal to each other, not the times.

Step 5 ：Distance = Speed * Time. For upstream, we have Distance = (x-7) * 30/60.

Step 6 ：For downstream, we have Distance = (x+7) * 20/60.

Step 7 ：We can set these two distances equal to each other and solve for x.

Step 8 ：Final Answer: The boat speed in still water was \(\boxed{35}\) km/hr.