$1.$ A customer who wants a coffee moves sequentially to $2$ stations staffed by $4$ employees in a Starbucks:

The customer first moves to one of the two cash registers where Alex and Bob are working. One of them places the order and accepts payment. There is one extra cash register where no one is working.

One of Charlotte and David makes the coffee using a coffee machine. Both Charlotte and David have their own coffee machines. There is o