The force applied on a body of mass 5kg is 20 N. If the body moves through a distance of 12 m, calculate the amount of work done.

In physics, work occurs when a force is applied to an object which then moves (has displacement) in the same direction as the force.  The amount of work that is done in the situation in your question is 240 J where “J” stands for joules.  We find this value by using the equation W = Fd where W is work, F is force, and d is displacement.
In your question, only the force applied to the object and the distance it moves (is displaced) are important.  The weight/mass of the object is irrelevant and is probably included to test whether you understand that it is not used in calculating work in this case.  To illustrate, let us look at a very similar example problem from the physics curriculum that I am teaching from this semester:
In this problem, a woman lifted a 40 kg bag to a height of 0.8 m. To do this, she used a force of 500 N. The question asked how much work she did in lifting the bag.  The correct answer was:
W = Fd = (500 N)(0.8 m) = 400 J
In your question, the equation would look like this:
W = Fd = (20 N) (12 m) = 240 J
One way to be certain that you should not have used the mass of the object in your equation is to look at the units involved.  A newton is defined as kg*m/s^2.  When you multiply newtons by meters, you get kg*m^2/s^2.  This is the definition of a joule.  If you included the mass of the object, you would square the kg as well and you would not end up with an answer in joules.  Since work is measured in joules, you would have made a mistake.
So, in this problem, disregard the mass of the object, use the equation W = Fd, and find that the work done in this instance was 240 J.
https://www.physicsclassroom.com/calcpad/energy