if a porter of height 150cm lifts the load 20kg from railway platform and keeps on his head,the work done by gravity on load is--------------.

As the displacement is taking place in opposite direction to the gravitational force, it is negative.

Disp = - 150 cm = - 1.5 m

Work Done = Force x Disp = m g x disp = 20 kg x 10 m/s² x - (1.5 m) = [Calculate yourself]