3
Review: measuring, mass and time
Discover Mathematics P2: Term 1, Lessons 2–60
GoalReview measuring with a ruler, choosing g or kg, mass problems, and telling time.
New words
centimeter · a small unit of length, cmkilogram · a big unit of mass, kgminute hand · the long hand on a clocka.m. and p.m. · before noon and after noon
Explanation
When you measure length, start at 0 on the ruler and read where the object stops. 100 centimeters make 1 meter. Choose g for light things and kg for heavy things, and write the unit in your answer. When you tell time, read the hour hand first and then the minute hand. Count by fives: the minute hand on 3 is 15 minutes (quarter past), on 6 is 30 (half past), on 9 is 45 (quarter to). Use a.m. for morning and p.m. for afternoon and night.
| We measure | Units | Tool |
|---|---|---|
| length | cm, m | ruler |
| mass | g, kg | balance |
| time | minutes, hours | clock |
Worked examples
The pencil starts at 0 and stops at 7. It is 7 cm long.
A bag of lemons is 18 kg and a bag of limes is 7 kg. 18 + 7 = 25 kg in all.
The minute hand is on 9 and the hour hand is nearly at 5. It is quarter to five, 4:45.
Class activity
Measure my morning. Measure the length of your bed in steps and then in meters with a long string. Weigh your school bag on a kitchen scale if you have one. Write the times of your morning routine with a.m. Which one needs cm or m, which kg, and which minutes?
Practice
1
How long is the line?
10 cm.
2
Which unit is best for a bag of potatoes from the market: g or kg?
kg.
3
A box has 70 g of salt. Nour uses 25 g. How many grams are left?
70 − 25 = 45 g.
4
What time does the clock show?
7:20.
5
School assembly is at the time shown, in the morning. Write it with a.m. or p.m.
8:00 a.m.