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(1) After removing the gravity and friction of pulleys and ropes, the pulling force exerted by each hand is f =12g =12× 4× 50n =100n;

(2) The work done by pulling each hand once w = fs =100n×1.6m =160J,

The total work w that people do per minute is always 2× 30× 160J = 9600J.

The average power of people who do work on sports equipment is P=W, and the total t = 9600J60s =160 w;

(3)√η= W Total useful w =GhF2h=G2F

∴ pulling force F'=G2η=4×50N2×80%= 125N,

So the increased tension of each hand △ f = f ′-f =125n-100n = 25n.

Answer: (1) Then the pulling force exerted by each hand is100n;

(2) The average power of people working on sports equipment is160 W;

(3) The pulling force of each hand should be increased by 25 N. 。