If they're both in the same day, then it's 71/4 hours.
9 am to 4:15 pm is approximately 7 hours and 15 minutes.
8 hours and 15 minutes.
12 hours.
21
From 9am to 6:30pm is 9 hours and 30 minutes.
5 Hours
6 hours
9 hours
21 hours
There are zero hours between 9 am and 9 am.
11 hours