Let's compare the cost of both the centers.
Cost of Center A:
One time charge of $495 in one year.
Cost of Center B:
- Flat $25 dollar sign up fee
- $15 per month, so 
  dollars a year dollars a year
- $5 per aerobic class, so 
  dollars a year (given that Billy goes to class once a week) (Note: There are 52 weeks in a year) dollars a year (given that Billy goes to class once a week) (Note: There are 52 weeks in a year)
Total cost = 
 dollars
 dollars
Hence, Center B would cost 
 dollars cheaper.
 dollars cheaper.
ANSWER: Least expensive club for Billy to use for a year is Center B