How many of these problems do you have?

11:30 = 11 hrs 30 min

9:45 = 9 hrs 45 min

You can't subract 45 min from 30 min, so you borrow an hour from 11 and tack on the 60 min to the 30 min you already have:

10 hrs 90 min - 9 hrs 45 min = 1 hrs 45 min