10 hours and 45 minutes.
7:45 am to 6:15 pm is 10 hours 15 minutes (101/4 hours)
From 8am to 6:15pm is 10 hours and 15 minutes.
There are 10.25+24k where k is an integer.
From 7:45 AM to 1:15 PM is a total of 5 hours and 30 minutes. To break it down, there are 5 hours from 7:45 AM to 12:45 PM, plus an additional 30 minutes to reach 1:15 PM.
6:15am to 5pm is 10 hours and 45 minutes.
8.5 + 24k hours where k is any integer.
From 6:15am to 1:30pm is 7 hours and 15 minutes.
There are 9 and 1/2 hours between 8:45 am and 6:15 pm.
IF they're both on the same day, then they're 5hours 10minutes apart.
13 + 24k hours where k is an integer.
From 6:15pm to 7:15am is 13 hours.