For every ticket purchased, the cost will increase by $15.00 because x indicates that $15 is the unit rate for each ticket.
The quickest way to get the series is if you already know the series for sine:

Then just multiply the series by
to get

The first four nonzero terms are simply the first four terms:

Associative property is your answer
The Associative property states that no matter which group is parenthesized, the answer will still be the same.
hope this helps