Problem reads,

A man of mass 60 kg, standing on a light weighing machine kept in a box of mass 30 kg. The box is hanging from a pulley fixed to the ceiling through a light rope, the other end of which is held by the man himself. If the man manages to keep the box at rest, what is the weight shown by the machine? What force Should he exert on the rope to get his correct weight on the machine?

Solution) Let the man exets force F in the state when he manages to keep the box at rest. in this situation his effective weight as shown by the machine will be  = (mg - F), where m = mass of the man = 60 kg

Now, for this we formulate following equation

weight of box+(weight of man - force applied by man)=Force applied by the man

or, 30g + (60g - F) = F

or, 90g = 2F

or F = 45g

Therefore weight of the man as applied on the machine = (mg - F)

                                               = 60g - 45g = 15g

thus, weight applied on the machine = 15g Newton

Which corresponds to mass = 15 kg