You can read a clock to the minute. Elapsed time is how much time passes from a start time to an end time: how long recess lasts, or when a movie ends.
Why it works
An hour has 60 minutes, not 100, so times cannot be subtracted like ordinary numbers. Instead, count up. Jump from the start time to the next o'clock, then jump on to the end time, and add the jumps. The o'clock is a friendly stepping stone, like a ten on the number line.
Key idea
Count up from the start to the next hour, then on to the end. Add the jumps. To find an end time, add minutes the same way: first up to the hour, then the rest.
Worked example: Across the hour
Recess starts at 9:50 and ends at 10:10. How long is it?
- 9:50 to 10:00 is 10 minutes.
- 10:00 to 10:10 is 10 minutes.
- $10 + 10 = 20$.
Answer: 20 minutes
Worked example: Find the end
A movie starts at 2:35 and lasts 50 minutes. When does it end?
- 2:35 to 3:00 is 25 minutes.
- $50 - 25 = 25$ minutes are left.
- 3:00 plus 25 minutes is 3:25.
Answer: 3:25
Worked example: Find the start
Practice ended at 4:10. It lasted 25 minutes. When did it start?
- Go back 10 minutes from 4:10 to 4:00.
- $25 - 10 = 15$ more minutes back: 15 minutes before 4:00 is 3:45.
Answer: 3:45
Common mistake
Subtracting times like ordinary numbers. $1010 - 950 = 60$, but 9:50 to 10:10 is only 20 minutes. The mistake treats an hour as 100 minutes. Count up through the o'clock instead.