You have to know the center of gravity. I do not know what a,b, c mean above.
Sum moments around any point. Sum vertical forces. Those two equations should do it.
A man doing push-ups pauses. His mass is 80-kg. Determine the normal force exerted by the floor on each hand and on each foot.
a = 40 cm
b = 95 cm
c = 30 cm
W = mg = 784 N
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