Question

if action is always equal to the reaction , explain how much a horse can pull a cart

Answer 1

Acton is always equal to reaction but they work on different objects. horse pushed the Earth in backward direction so Earth pulled it in forward direction. therefore horse can pull a cart.

Answer 2

When horse pull a cart with some force cart also apply same force acc. toN's 3rd law. these force cancel out due to apposition direction but horse also push earth with his feet but  earth has large mass earth does not move backward .acc. . to N's 3rd law earth also push the horse in forward  direction as the result of it horse move in forward direction