Measurement and Money
Primary 5 · Mathematics
Measurement and Money
Lesson 2 of 3
15 min
Let's begin
Gatete is at Nyabugogo bus park with a ticket in his hand. The ticket says the bus to Huye leaves at 14:30.
He looks at the clock on the wall. It only goes up to 12. There is no 14 anywhere on it. His heart beats fast - has he already missed the bus?
No. He has just met the 24-hour clock. By the end of this lesson you will read it as easily as you read your own name.
The 12-hour clock counts 1 to 12, then starts again. Because it repeats, it must tell you which half of the day you mean:
The 24-hour clock never repeats. It counts the hours straight through, 00 to 23, always with four digits, and it never needs a.m. or p.m.
It is 2:30 in the afternoon. Write it the 24-hour way.
Afternoon means p.m. The morning hours have already used up 12 hours, so add those 12 on:
2 + 12 = 14. The minutes never change.
2:30 p.m. = 14:30. That is Gatete's bus.
A bus arrives at 19:45. Say it the 12-hour way.
19 is bigger than 12, so this moment is after midday. Take the 12 back off:
19 − 12 = 7.
19:45 = 7:45 p.m.
School assembly is at 09:15. The hour 09 is smaller than 12, so it is still morning. Change nothing, just add the label.
09:15 = 9:15 a.m.
p.m. going into 24-hour time: add 12 to the hour.
Hour bigger than 12 coming back: subtract 12, and call it p.m.
Morning hours stay exactly as they are.
Midnight is 00:00. The day starts from zero. So 00:45 is forty-five minutes after midnight, in the deep dark.
Midday is 12:00. So 12:45 is not in the night - it is a quarter to one in the afternoon, when you are probably eating lunch.
60 seconds = 1 minute | 60 minutes = 1 hour | 24 hours = 1 day
Worked example
How many minutes are in 2 h 15 min?
Each hour is 60 minutes: 2 × 60 = 120 minutes. Then add the loose 15.
120 + 15 = 135 minutes.
Worked example
Umutoni revised for 200 minutes. How many hours and minutes is that?
How many whole 60s are in 200? 60, 120, 180 - that is three, and 20 minutes are left over.
200 minutes = 3 h 20 min.
Worked example
A football match kicks off at 15:40 and the referee blows the final whistle at 17:25. How long was the match?
Hop to the next whole hour first, then take big jumps, then finish the small bit.
Now gather them up: 1 hour, and 20 + 25 = 45 minutes.
The match lasted 1 h 45 min.
Check it by subtracting in columns. You cannot do 25 - 40, so borrow one hour from the 17 - but one hour is 60 minutes, not 100. That gives 16 h 85 min.
16 h 85 min - 15 h 40 min = 1 h 45 min. The same answer.
Umutoni's science lesson starts at 08:50 and lasts 40 minutes. When does it end?
From 08:50 to 09:00 uses 10 of the 40 minutes. That leaves 30 minutes.
09:00 plus 30 minutes = 09:30. The lesson ends at 09:30.
Warning
Time is not money
With francs you borrow 10. With time you borrow 60, because an hour has 60 minutes, not 100.
Anyone who works out 17:25 - 15:40 as if the digits were plain numbers gets the silly answer 1 hour 85 minutes. But 85 minutes is more than an hour! If your minutes come out at 60 or more, you have made this mistake.
A timetable is just a table of times. Each row follows one bus. Each column is one place along the road.
| Bus | Kigali | Muhanga | Huye |
|---|---|---|---|
| Bus A | 06:00 | 07:05 | 08:30 |
| Bus B | 09:15 | 10:20 | 11:45 |
| Bus C | 13:30 | 14:35 | 16:00 |
Worked example
How long does Bus B take from Kigali all the way to Huye?
Find the Bus B row. It leaves Kigali at 09:15 and reaches Huye at 11:45.
09:15 → 11:15 is 2 hours. 11:15 → 11:45 is 30 minutes.
The journey takes 2 h 30 min.
Worked example
Keza must be in Huye before 12:00 for a family gathering. Which is the latest bus she can take?
Look only at the Huye column, because that is what matters.
The latest bus that still works is Bus B. Choosing well is part of mathematics too.
Worked example
How long is Bus A's journey from Muhanga to Huye?
Read across the Bus A row: 07:05 at Muhanga, 08:30 at Huye.
07:05 → 08:05 is 1 hour. 08:05 → 08:30 is 25 minutes.
That part of the trip takes 1 h 25 min.
Tip
How to remember it
For the clock: after lunch, add 12. 1 p.m. is 13:00, 6 p.m. is 18:00, 9 p.m. is 21:00.
For duration: hop, jump, step. Hop up to the next o'clock, jump the whole hours, then step out the last minutes. Add up your hops, jumps and steps and you have the answer - and you can never go wrong with borrowing.
Use the bus timetable above for questions 5 and 6. Answers are underneath.
What you learned today
Quick check
Match each 24-hour time to the same time on the 12-hour clock.
Tap one on the left, then its partner on the right.
