Assignment:
A certain city has experienced a relatively rapid increase in traffic congestion in recent times. The mayor has decided that it's time to do something about the problem and decides to form a task force to research the causes of the problem and come up with suggestions for dealing with it. The mayor first compiles a list of names of 21 residents who will be considered for membership on the task force. Twelve of those 21 people are Republicans, and the other 9 are Democrats. How many ways are there to form a task force of 7 people from that list of 21 candidates if at least one member of the task force has to be a Republican and at least one member has to be a Democrat?
Provide complete and step by step solution for the question and show calculations and use formulas.