How to teach elapsed time with a number line
Help your child calculate elapsed time by counting forward through the hour, with a number-line example, practice questions and clear answers.
Your child reads digital times and knows an hour has 60 minutes.
Teach elapsed time by counting forward from the start time to the finish time. When the interval crosses an hour, stop at the next o'clock time, then continue. A number line records those jumps so your child can add the minutes that have passed.
Start with two times on the same afternoon. Once that feels comfortable, introduce longer intervals or times that cross midday.
Understand the idea
Elapsed time is how long an event lasts. The start and finish are clock times; the answer is a duration, such as 35 minutes.
An hour has 60 minutes. That means 3:59 pm is followed by 4:00 pm, not 3:60 pm. Clock notation needs different handling from ordinary decimal numbers.
Say: "We know when it started and when it finished. Let's count the minutes in between. Where is the next whole hour?"
If your child is still learning to read digital times, practise that separately first. You can move an analogue clock's minute hand alongside the number line to show how each jump changes the time.
See it worked through
Count across the hour
A game starts at 3:40 pm and finishes at 4:15 pm. How long does it last?
Draw a line with 3:40 pm at the left and 4:15 pm at the right. Mark 4:00 pm between them.
From 3:40 pm to 4:00 pm, 20 minutes pass. Explain that the minute count moves from 40 to 60, completing the hour.
From 4:00 pm to 4:15 pm, another 15 minutes pass.
Add the two durations: 20 + 15 = 35 minutes. The game lasts 35 minutes.
The line needs to show the events in time order. If you draw it to scale, the 20-minute interval should be longer than the 15-minute interval. When you make a quick sketch that is not to scale, rely on the labels to name each jump's duration.
Check by starting at 3:40 pm and adding 35 minutes. Twenty minutes takes you to 4:00 pm and the remaining 15 takes you to 4:15 pm.
Try it together
Try a new start and finish
A drawing activity starts at 10:45 am and finishes at 11:20 am on the same day. How many minutes does it last?
Ask your child to draw the start and finish on a line and choose an o'clock time between them. Let them label the minutes in each jump before adding.
Then ask: "If we know the activity starts at 10:45 am and lasts 35 minutes, what time will it finish?"
Check the answer
From 10:45 am to 11:00 am is 15 minutes. From 11:00 am to 11:20 am is 20 minutes. Together that is 15 + 20 = 35 minutes. Adding those 35 minutes to 10:45 am gives a finish time of 11:20 am.
Check their understanding
Ask your child to explain the difference between "4:15 pm" and "35 minutes". The first tells us a point in the day; the second tells us how long something lasts.
Try an interval within the hour: 2:10 pm to 2:35 pm. That lasts 25 minutes. Then ask about 2:50 pm to 3:05 pm. Crossing the hour gives ten minutes plus five, or 15 minutes altogether.
Make it easier or harder
Make it easier with 3:00 pm to 3:20 pm. Count in five-minute jumps, or move the minute hand on a clock. Then try 3:30 pm to 4:00 pm, which is 30 minutes.
Make it harder with 9:35 am to 11:10 am. The jumps are 25 minutes to 10:00 am, one hour to 11:00 am and ten more minutes. That is one hour 35 minutes, or 95 minutes. Ask your child to explain why both duration answers agree.
Teaching notes
The Victorian Curriculum's elapsed-time guidance describes using start and finish times to calculate durations. These original activities practise that skill by recording and combining forward jumps.
Numeracy practice
Help your child apply their maths understanding with practice questions and explanations.
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