Uniform plank 5m rests horizontally on 2 bricks that are 0.5m from each end A child weighing 200N stands on 1 end of the plank and causes the other end to lift Weight of plank?

We have to assume that the question is saying that the 200-N is the minimum force required to cause the plank to just barely rotate. When the rotation begins, we have a simple lever on one brick, and we can forget about the second brick. The child is standing on the lever's short arm. The child's first moment is (200N x 0.5m), and the first moment of the short piece of plank is (0.1W x 0.25m). The sum of moments on the short side is (100 + 0.025W). On the long side, the moment is (0.9W x 2.25) = (2.025W) . The moments on both sides are equal ===> (100 + 0.025W) = (2.025W) 100 = 2W ===> W = 50N