Change-making feasibility

Smoothies cost exactly 5 dollars at a street stand. Customers pay one at a time, each with a single bill: a 5, a 10, or a 20 (given in order as bills). The stand opens with no cash and must give every customer exact change using bills collected so far. Return True if every customer can be served in order. Example: bills [5, 5, 10, 20] is True.