Your "total hours" might be 10:00, but if you select that cell it will most likely show "10:00:00 AM" in the function bar above.

Times are stored as fractions of 1 day, so all times are between 0 and .999. If you format a cell that has a time in it as a number with decimals, you'll see the decimal equivalent of the time shown.

10:00:00 AM is equivalent to 0.416667.
10:30:00 AM is equivalent to 0.4375.

If you multiply either by 24 you will get the correct number of hours for your calculation.

0.4166667 * 24 = 10
0.4375 * 24 = 10.5
etc..

As mentioned in a previous, although now missing, post, you could also use

=HOUR(A1)

where A1 holds the total time in hh:mm format.

=HOUR(A1) where A1 = 10:00 would result in just 10.