There's another way to get to decimal hours from a time of day difference that predates the introduction of the Duration format in Numbers. TIMEVALUE gives you the fractional day associated with a time of day, regardless of the date. (There can be an advantage to using TIMEVALUE if time of day values are entered on a different day than the day that the cell represents.) To do your calculate using the Date-independent TIMEVALUE function, you could write:
=(TIMEVALUE(C6) - TIMEVALUE(B6)) * 24
The multiplication by 24 converts from decimal day difference to decimal hours difference.