The time between a start and an end is an interval.
Interval is how long something lasts, from start to finish.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
Count on, do not subtract
From 3:40 to 4:15, count on to 4:00 first: that is 20 minutes. Then 15 more.
20 and 15 makes 35 minutes.
Why counting on is safer
Clock numbers do not subtract like ordinary numbers, because an hour is 60, not 100.
Finding how long something took
From 2:15 to 3:40, count up in convenient pieces: 2:15 to 3:00 is 45 minutes, and 3:00 to 3:40 is 40 more. Total 85 minutes, which is 1 hour 25 minutes. Counting up through the whole hour is far safer than trying to subtract times in columns.
Time does not work in tens
You cannot do 3:40 - 2:15 the way you would do 340 - 215, because an hour is 60 minutes, not 100. This is the single biggest source of error in time problems. Whenever you regroup with time, you exchange one hour for 60 minutes.
Working out a start or end time
If a film lasts 95 minutes and ends at 4:20, count back: 95 minutes is 1 hour 35, so go back an hour to 3:20 and then 35 minutes more to 2:45. Breaking the interval into hours and minutes makes counting backwards manageable.
Step 2: Try It Yourself
Jump to the hour first, then jump the rest.
40 + 20 + 15 = 60
The round dot is where you started. The square is where you landed.
Step 3: Watch an Example
One step at a time.
Watch Ana Count On
A film starts at 3:40 and ends at 4:15.
- Step 1
She counts from 3:40 up to 4:00, which is 20 minutes.
Step 4: Your Turn
Practice makes it stick.
Break Time
Problem 1 of 2
Break starts at 10:45 and ends at 11:05. How long is it?
minutes
The Lesson
Problem 2 of 2
A lesson runs from 2:20 to 2:55. How many minutes is that?
minutes
How Long?
1 of 3
From 1:10 to 1:45. How many minutes?
2 of 3
From 5:50 to 6:20. How many minutes?
3 of 3
From 9:35 to 10:05. How many minutes?
Step 5: Quick Check
Show what you know.
Question 1 of 2
From 7:45 to 8:10. How many minutes?
Question 2 of 2
Why count on to the hour first?
What You Learned
- An interval is how long something lasts.
- Count on to the next hour, then count the rest.
- Add the two jumps to get the total.