ATMs and, in general, any kind of machine that accepts/returns banknotes or coins (for example, a snack-vending machine) use the Chain of Responsibility pattern.
There is always a single slot for all banknotes, as shown in the following diagram, courtesy of sourcemaking.com (www.sourcemaking.com):
When a banknote is dropped, it is routed to the appropriate receptacle. When it is returned, it is taken from the appropriate receptacle. We can think of the single slot as the shared communication medium and the different receptacles as the processing elements. The result contains cash from one or more receptacles. For example, in the preceding diagram, we see what happens when we request $175 from the ATM.
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