m = 25.5 kg, the mass of the child
h = 2*20 = 40 cm = 0.4 m
g = 9.8 m/s², acceleration due to gravity.
The work done is equal to the increase in PE (potential energy).
Therefore the work done is
W = (mg)*h
= (25.5*9.8 N)*(0.4 m)
= 99.96 J
Answer: 99.96 J