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 one extra coffee machine which is not used by anyone. a coffee is $60$ seconds. ${?}_{(})$

$\backslash$table$[[$Tasks$,$Task time$],[$Place an order & accept payment,$45$ seconds$],[$Make a coffee,$60$ seconds$]]$

Note: Alex & Bob take the same amount of time to do their tasks.

Charlotte & David take the same amount of time to do their tasks.

$($a$)$ Draw the process flow diagram. $(3$ points$)$