Adding a formula that calculates dates by repair hours

I have 4 columns
1= customer
2= date arrived
3= repair hours
4= date must be completed

I'm trying to get column 4 to be repair hours divided by 5 (work hour goal per day) but also factor in the date arrived then automatically calculate the date it must be done to acheive a "5" hour per day.

Any help would be much appreciated.

Thanks!

Dave