12am to 4pm is 16 hours.
From 12am to 8am is a total of 8 hours. This is because there are 8 hours between the two times, counting inclusively. To calculate this, you can subtract 12 (midnight) from 8 (8am) which equals 8 hours.
12 hours
18 hours
21
17
4
7
21
10
113 hours
72 hours from Sunday at 12am would be Wednesday at 12am. This is because there are 24 hours in a day, so three days would be 72 hours. Starting at Sunday at 12am, you would count 24 hours for each day, bringing you to Wednesday at 12am.