Can I convert a pick-list number and convert it to an integer?

Can I convert a pick-list number and convert it to an integer?

I'm trying to create an invoice from a case. Basically a service guy inputs the number of hours worked from a picklist. When an invoice checkbox is ticked an invoice is created.

I've got most of it working but my problem is I don't know how the number of hours selected from the piccoloist can be converted to an int to do calculations on.

Summary:
- Case is opened for a client
- Tech does work and selects number of hours worked from a piccoloist
- Ticks a box to create the invoice
- On invoice creation (via custom function) I want to calculate serviceRate * numberHours

Problem:
- numberHours is a string.
- how do I convert it to a number?