Pressure is force per unit area. To calculate the pressure exerted on the ground by a person, we need to know the force the person is exerting and the area of their feet.
The person weighs 70 kg, and the force of gravity acting on them is equal to their weight, so the force exerted on the ground by the person is 70 kg x 9.8 m/s^2 = 686 N (Newtons)
The area of their feet is 400 cm^2, which is equal to 0.04 m^2 (since 1 cm^2 = 0.0001 m^2)
The pressure exerted on the ground by the person is equal to the force exerted divided by the area of the feet:
Pressure = 686 N / 0.04 m^2 = 17150 Pa (Pascals)
If the man stands on one foot, the area of the foot in contact with the ground will be half the original area, so 0.04 m^2/2 = 0.02 m^2
The pressure exerted on the ground will be 686 N/ 0.02 m^2 = 34,300 Pa. So he will exert double the pressure on the ground as he was exerting before.