How to get time difference in hours and fractions of hours?

example 3 columns in a spread sheet
Col A and B formated as "Time"
Col C formatted as Custom "h:mm" produces result 0:20 in example below
how to format so as to produce result 0.3333?
I tried formatting as "h" but it then produces 0
formatting as 0.00 produces 0.01 - no clue how it comes up with that one?

Col A Col B Col C
Start Time End Time Elapsed
2:50 PM 3:10 PM = B-A

thanks
mark