Gaurav ;

According to D'Alembert , a fictional force or a pseudo force acts in a direction opposite to that of the motion of the free body. It amounts up as an amount of it's mass multiplied by it's acceleration.

So, in this case the mass of the freely falling body is 20 kg and the acceleration is 6m per sec² . So, the fictional force will be 20*6 = 120 N .

Hope you got your answer!