1Units of time come in groups of 60
1 minute is 60 seconds, and 1 hour is 60 minutes. The numbers we normally use carry into the next place every 10 — but time is different, and only carries over every 60.
Seconds → Minutes60 seconds make 1 minute
Minutes → Hours60 minutes make 1 hour
When minutes reach 60, they reset to 0 and the hour goes up by 1
2The clock hands move at three different speeds
The second hand makes a full turn every 60 seconds, the minute hand every 60 minutes (one hour), and the hour hand every 12 hours. That's why the second hand looks like it's racing around, while the hour hand barely seems to move.
3Elapsed time is "carry over whatever spills past 60"
To find the difference between two times, whenever seconds or minutes go past 60, just carry that amount up to the next place.
From 3:40 to 4:15 — how many minutes passed?
From 3:40, 20 minutes bring you to 4:00, and then 15 more minutes pass — so a total of 35 minutes went by.
Sometimes you need to borrow, too.
If the later time's "minutes" are smaller than the earlier time's "minutes," borrow 1 hour (60 minutes) from the hour and do the subtraction. It's the exact same idea as borrowing in place-value subtraction.
If the later time's "minutes" are smaller than the earlier time's "minutes," borrow 1 hour (60 minutes) from the hour and do the subtraction. It's the exact same idea as borrowing in place-value subtraction.
4Try playing like this
- Move the hour, minute, and second sliders. Watch how the minute hand jumps the moment the seconds reach 60, and how the hour hand jumps the moment the minutes reach 60.
- Pick two times yourself. Choose two times that cross an hour mark, and practice calculating the elapsed time.
- Check your work with the quiz. Compare your own answer with the quiz's correct answer to get a better feel for when borrowing is needed.