What would happen to the inductance of a solenoid if you doubled it's length and doubled the total number of turns.

To solve the problem it is necessary to apply the concepts of Torque and equilibrium.

The Torque is defined as:

Where

F= Force

d = Distance

In this particular case, the force is caused by the weight of both children.

In turn, when there is equilibrium, the two torques must be equal therefore

Replacing with our values

Re-arrange to find  

Therefore the distance that the second kid should sit to balance the see-saw is 1.8m from the pivot.