Here are two bus times from a Dublin Bus timetable. One bus leaves your stop at 8:47 and the very next one reaches the school gate at 9:25.

How long is that journey? Don't work it out fully yet, just have a guess in your head: is it more or less than half an hour?

8:47 to 9:25

Elapsed time just means how long passed between leaving and arriving. The clever trick is to count on in two hops: first up to the next o'clock, then on to the arrival time.

Let's build the first hop, from 8:47 up to 9:00. Count on in small jumps: 47, then 50 is 3 minutes, then 55 is 5 minutes more, then 60 (which is 9:00) is another 5. That is 3 + 5 + 5 = 13 minutes to reach 9 o'clock. Now the second hop, from 9:00 on to 9:25, is 25 minutes. So 13 + 25 = 38 minutes in total.

Picture it on a number line: a small hop of 13 minutes lands us on the 9 o'clock mark, then a bigger hop of 25 minutes lands us on 9:25. Two hops, one friendly stop in the middle.

10:15 to 10:55

This time both times sit inside the same hour, so there is just one easy hop: from 15 minutes past to 55 minutes past is 40 minutes.

6:30 to 7:00

A nice clean half-hour. From half past six to seven o'clock, how many minutes is that?

Today we work these journey times out together. I'll set a start time on the clock, then call out an arrival time, and you count on to find how long the journey lasted.

Today we work through four bus-journey times together: 10:20 to 10:50, 7:15 to 7:45, 8:50 to 9:20, and 11:48 to 12:25. For each one, count on in two hops and predict how long the journey took before we move the hands. Then set the clock to the arrival time and use the elapsed readout to check whether your count was right.

What we learned today

• Elapsed time is how long passes between a start and an end time.
• Counting on in two hops — up to the next o'clock, then on to the end — makes the count easy.
• The o'clock is the friendly stop that splits a tricky journey time into two simple parts.