Answer: 3920 N
Step-by-step explanation:
The force acting on his legs is equal to the resultant total force on his body I.e his weight plus the mass he's carrying.
M = m1 + m2
M1 = mass of the weight lifter
M2 = mass of the weight he's carrying
M = sum of the total mass acting on body
M = (100 + 300) kg
M = 400kg
But force = mass * acceleration due to gravity,
Force (f) = 400 * 9.8
F = 3920N