Explanation:
Remember that power is defined as how much Work is done per unit of time.
Work is defined as the amount of force applied across a certain distance.
Since in both cases of climbing the ladder (on Earth and the moon), Luke coveres the same amount of <em>distance </em>in the same amount of <em>time</em>, we are only left with one difference between the two cases - gravity.
If you were to carry your backpack on the moon with the same load of text books, it would take less force to pick it up on the moon. Therefore, Luke expends less effort on the environment with less gravity - the moon.
To find the difference factor - you would want to divid the gravitational contants between earth and the moon.