The mechanical advantage is the ratio of the output force and that of the input force. Machines are used with a mechanical advantage greater than 1 as it is easier to apply a small force over a prolonged displacement rather than a very large force over a small distance. It should be noted that the output energy cannot be larger than the input energy.
Here, a pulley is used to apply a force of 50 N which allows a mass of 20 kg to be lifted. The mass of the box is 20 kg and there is a gravitational force acting on it that accelerates it downwards with an acceleration equal to 9.8 m/s^2
The weight of the box is 20*9.8 = 196 N
The mechanical advantage here is 196/50 = 3.92.