It is often easiest to use "military time". That is, add 12 to all the afternoon numbers and do the subtraction in the usual way. Of course, 1 hour = 60 minutes, so 10 minutes = 10/60 hour = 1/6 hour.
Mon: 15:10 -8:00 = 7:10 = 7 1/6
Tue: 15:25 -8:05 = 7:20 = 7 1/3
Wed: 14:30 -8:00 = 6:30 = 6 1/2
Thur: 14:45 -7:55 = 7:(-10) = 6:50 = 6 5/6
Fri: 15:38 -7:58 = 8:(-20) = 7:40 = 7 2/3
_____ Some calculators have nice features for working with degrees, minutes, and seconds. In this context, degrees and hours are the same thing. That is, the base-60 arithmetic is the same whether you consider the numbers to be hours or degrees. Similarly, some calculators convert nicely between decimal fractions and mixed numbers. In short, a suitable calculator will almost do this math for you. (You just need to add 12 to all the numbers in the column on the right.)
3 places. 6 options in the first place, 5 options in the second place, and then 4 in the last place. Because one option is used every time you pick one.
