A 30-kg child sitting in a chair applies a force of approximately 300 N downward. on the chair approximately how much force does the chair apply upward to the child A. 10N
B. 30N
C. 300N
D .600N

moment's equilibrium required

moment = force x perpendicular distance from force's line of action (from pivot)

counter clockwise = clockwise moments

=> 15g(1.5) = F(0.3)

=> F = 15*9.81*5 = 735.75 ~= 736 N

Explanation:

im sorry i dont know

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.