The correct answer is 60 kg. The sum of the torques (force X Distance) has to be zero for the lever to be in balance. Use one direction to be negative (say, left of the fulcrum). Therefore (300 x -1) + (Box B x 5) = 0 Solving for Box B gives 60 kg. In reality, we are dealing with forces, so we would have to multiply both masses by 9.81 which is acceleration due to gravity. In this problem that would not affect the outcome for the mass of Box B. It would still be 60kg. So, you are correct, the correct answer is not listed.
