Use decimal type for money related exercises

Any exercises related to money should use the `decimal` type instead of `double`or `float` (see [this page](https://stackoverflow.com/questions/693372/what-is-the-best-data-type-to-use-for-money-in-c). Out of the top of my head, these exercises will include (but there may be more):

- book-store
- bank-account

