The first problem's answer is 6 3/4 hours because if you take away 45 minutes from 4:15 you will get 3:30. So if you add 6 hours 8:45 you will get 2:45, so then all you have to do is add another 3/4 hour, so the answer to no. 1 is 6 3/4 hours.
The answer to the second problem is 66 7/8 because if you change 68 1/2 to 68 4/8 then you subtract 1 you get 67 4/8. Then you subtract 4/8 leaving you with an additional 1/8. You then have 67. Then you subtract 1/8 from 67 and you get 66 7/8.
