This is because from 10:45 to 11:00 is 15 minutes and then another 15 minutes is 11:15 which adds up to 30 minutes and that tells you how much time has passed from 10:45 to 11:15
We can break down how to find elapsed time using a number line. We write 10:45 on one side and 11:15 on the other. Then we do a sort of guess, check, revise until you get the elapsed time.
