A seesaw is balanced with a 40 kilogram child sitting 2 metres from the fulcrum. Where must a 20 kilogram child sit on the other side to balance it?

Correct answer to question 133 (Mechanical Comprehension): 4 metres from the fulcrum

Method

A lever balances when weight times distance is equal on both sides: 40 x 2 = 80, so the lighter child needs 80 / 20 = 4 metres.