When students are dealing with calculations in time, the biggest mistake they make (over and over) is to try and use 'normal' addition and subtraction.
Time calculations are based around the number 60. There are 60 minutes in an hour and 60 seconds in a minute. (Normal calculation is based around 10, 100 etc)
The easiest way to do this is by counting on to the hour. If this method is used, students are much less likely to get answers that cannot be true.
Example 1: A TV programme starts at 09:00 and lasts 75 minutes. What time does it finish?
If we start at 09:00 and add 60 minutes we get to 10:00
This leaves us with 15 minutes so the programme ends at 10:15
Example 2: A 50-minute tv show ends at 15:45. What time did it start?
For this one, we're going backwards so we count down,
If I count down from 15:45 to 15:00 we have used 45 of the minutes
We just need to take 5 minutes off. This means the programme must have started at 14:55
Example 3: The winner of a formula one race has a time of 1 hr 58 minutes. If the second-place finisher has a time of 2 hrs 4 minutes, how long after the winner did the 2nd place finish?
This is easier than it looks, all we want is the difference between 1 hr 58 mins and 2 hrs 4 mins.
If we count on to the hour from 1 hr 58, it takes 2 minutes to get to 2 hours
We can then add the 4 minutes to get to 2 hrs 4 minutes. This gives us a difference of 6 minutes.