how do i subtract date and time into hours and convert hours to decimal in excel

I am trying to subtract cells that contain a date and time from another cell that contain a date and time into hours and then convert that to a decimal. E.g. 18/10/2018 4:53:50 AM minus 17/10/2018 5:12:30 PM equals (hours as a decimal number). Anyone any ideas how I can reliably do this? Thanks in advance.