Can anyone help ?

I have a window cleaning round and Im trying to work out the formular required for working out my hourly rate for particular jobs.

I will have the following information

For example a job that takes 23 minutes charged at £12.50

- The Time a job takes to complete in minutes
- The cost of the Job in £s

all i need to know is the formular for calculating the hourly rate !

Im sure this will be an easy one and hope someone can help me out ...