Since, the weight of the person on planet A is half that on the earth, the acceleration due to gravity there will be 12 that on the earth. Hence he can jump double the height with the same muscular force.
Or
The potential energy of the person will remain the same on the earth and on planet A.
Thus, mg1h1=mg2h2
g1=accelereation due to gravity on earth=g
g2=accelereation due to gravity on planet A
h1=height on the surface of the earth
h2=height on the surface of planet A
If g1=g
then g2=12g
h1=0.4 m
Then h2=g1h1g2=g×0.4g2
or h2=0.4×2=0.8 m