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there are 56 shopping carts in a supermarket
The rather obvious answer is - so they can pick up more shopping than if they had to carry everything to the checkout !
Occasionally individual stores will offer a "reward" for the return of "lost" shopping carts, but this is not a matter of routine.
Usually they are called Baggers or Lot Attendants
No, gold cannot be found in shopping carts. Gold can be in other places, but it cannot be found in shopping carts.