excel formula help

[Magic_x_uk]

Hi, cannot seem to get my head round the following. perhaps some one can help with the formula for Excel.

here is what i want

basically. in what cell time you arrive at work. another cell time you leave work. then a way of adding those cells up.

eg:
A1........................B1................C1
Start Time........Finish Time.....Total Hours
8.30am...............3.30pm...........7 hours




Thank you people