This is a question relating to turning forces, known as *moments*. For the seesaw to be balanced the moment caused by the 50kg boy must be equal and opposite to the moment caused by the 40kg girl. A moment is given by the equation M (moment, Nm) = F (force, N) x d (distance from pivot, m). To calculate the force (F) resulting from the mass of each person, we must use newton's 2nd law (F = m x a) . The acceleration (a) here is equal to the gravitational acceleration (g) which is approximately 10N/s^{2} as given in the question. The force, or weight of the boy is therefore 500N (50 x 10), and the girl 400N (400 x 10). The moment of the boy can then be calculated using M = F x d, 1000 (moment) = 500 (force or weight) x 2(distance from pivot). The moment of the girl must be equal and opposite to this, so we have 1000 (moment) = 400(force or weight) x d (distance). If we divide this equation by 400 we get 1000/400 = d which gives the d = 2.5 m. The girl must sit 2.5 m from the pivot to balance the seesaw with the boy.