time as a fraction of hours

Is there an easy way to convert H:mm to a fraction of Hours
I would like to subtract a finish time from a start time and have the result as a fraction of hours.
Start 15:30 finish 18:45 total 3.25 hrs duration

Nick's picture

a1 15:30 a2 18:45 a3

a1 15:30
a2 18:45
a3 =(a3-a1)*24

format a3 as 0.00 and you will have: