If you format the column or cells as time (h:mm or h:mm AM/PM are both listed under custom formats), you can do the sums with them without needing to worry about the conversion to 60/24.
Looks like you're doing some kind of timesheet. I'd suggest one column for start, one for finish, and if you like you can add either a single one for lunch duration or two for lunch start & finish.
Formulas for these respectively would be:
With lunch duration
A1=start, B1=lunch duration, C1=finish
Total hrs formula would be "=(C1-A1)+B1" (without the quote marks)
With lunch start & finish
A1=start, B1=lunch start, C1=lunch end, D1=finish time
Total hrs formula would be "=(B1-A1)+(D1-C1)"
So long as you've formatted all the columns with the time format, and entered them in that format (using the colon) it should work.