Physics is needed for this challenge.
There is a lever. On one side is a bucket with negligible weight. On the other is a 10 kilogram weight. The bucket is a metre to the left of the pivot, the weight, 2 to the right. Lever has an additional 50 centimetres of unoccupied space. The weight of the lever is 1 kilogram per metre. The weight is attached to a string, and the string goes around a pulley. On the other side is a 7 kilogram weight. The challenge is to figure out how much water at 20° celsius must be poured into the bucket to balance the scale.