Separating overtime hours from regular hours in timesheet

Bit of a noob question i imagine.
If you look at my example photo you will see that this guy made 51 hours this week. I'd like to make it so that cell only shows a maximum of 40 hours. I want the OT Total cell to show all hours logged in beyond 40 hours. If someone could hold my hand through this i would greatly appreciate it.

Screen Shot 2017-04-20 at 9.28.40 AM.png

: [

Screen Shot 2017-04-20 at 9.58.56 AM.png