Thanks in advance for any help!

I am trying to calculate the average time it takes to accomplish a specific
task amongst many tasks during an 8 hour weekday, excluding holidays. Work
day is from 8 AM to 4 PM.

Model Date Received Date finished
Ford 10/17/07 8:30 AM 10/18/07 12:00 PM
Chevy 10/17/07 8:00 AM 10/17/07 12:00 PM
Ford 10/17/07 8:00 AM 10/17/07 12:00 PM
Chevy 10/17/07 8:40 AM 10/17/07 12:00 PM
Ford 10/18/07 9:41 AM 10/19/07 12:00 PM
Dodge 10/18/07 9:15 AM 10/18/07 12:00 PM


What formula would allow me to find out the average time it takes to process
a Ford?

Thanks again!