Measurement and Money
Primary 5 · Mathematics
Measurement and Money
Lesson 1 of 3
14 min
Let's begin
Mutesi's mother sends her to the shop with one instruction: Bring me two.
Two what? Two kilograms of beans? Two litres of oil? Two metres of cloth? Mutesi stands in the doorway and laughs, because a number on its own is useless.
Today you will learn to give every number the unit it deserves, and to change one unit into another without getting lost.
You already know these four length units. Look at how they grow:
They are joined together like this:
10 mm = 1 cm | 100 cm = 1 m | 1000 m = 1 km
A classroom door is 2 m tall. How many centimetres is that?
Think first: a metre is a big unit and a centimetre is a small unit. It takes many small ones to fill a big one. So the answer must be a bigger number than 2.
One metre is 100 cm, so two metres is two lots of 100 cm.
2 × 100 = 200 cm. The door is 200 cm tall.
Uwase measures her pencil with a ruler. It is 130 mm long. How many centimetres?
Now you are going the other way, from a small unit to a big one. You will need fewer of them, so the answer must be a smaller number than 130.
Every 10 mm makes 1 cm, so ask: how many tens are in 130?
130 ÷ 10 = 13 cm. The pencil is 13 cm long.
Ishimwe walks 2 km 350 m from his home to school. What is that in metres only?
The 2 km part: 2 × 1000 = 2000 m.
Now add the loose metres: 2000 + 350 = 2350 m.
You just did it three times, so you already know it:
Big unit into small unit → MULTIPLY. (the number gets bigger)
Small unit into big unit → DIVIDE. (the number gets smaller)
Mass is how heavy something is. Rice, beans and babies all have mass.
1000 g = 1 kg | 1000 kg = 1 tonne (t)
Worked example
A sack of rice has a mass of 25 kg. How many grams?
kg is big, g is small, so multiply: 25 × 1000 = 25000 g.
Worked example
A packet of sugar has a mass of 2500 g. Write that in kilograms and grams.
g is small, kg is big, so divide by 1000. How many whole thousands are in 2500? Two, with 500 left over.
2500 g = 2 kg 500 g.
Capacity is how much liquid a container can hold.
1000 ml = 1 litre (l)
Worked example
A jerrycan holds 20 l of water. Keza fills bottles that hold 500 ml each. How many bottles can she fill?
You cannot divide litres by millilitres, so make both the same first.
20 l = 20 × 1000 = 20000 ml.
20000 ÷ 500 = 40 bottles.
That is the golden move: same units first, then calculate.
Warning
The mistake almost everyone makes once
You cannot add 2 kg and 500 g and get 502 of anything. Kilograms and grams are different sizes, like adding cows and chickens.
Change them into the same unit first: 2 kg = 2000 g, so 2000 + 500 = 2500 g. Now it means something.
Worked example
(adding). Keza buys 1 kg 700 g of beans and 2 kg 500 g of maize flour. What is the total mass?
Start with the small unit, the grams: 700 + 500 = 1200 g.
But 1200 g is more than a kilogram! 1200 g = 1 kg 200 g. So write down 200 g and carry 1 kg across.
Now the kilograms: 1 + 2 + 1 (the carried one) = 4 kg.
Total = 4 kg 200 g.
Check it another way: 1700 g + 2500 g = 4200 g = 4 kg 200 g. The same. Good.
Worked example
(subtracting). A 5 litre jerrycan is full. Habimana pours out 2 l 400 ml for cooking. How much is left?
Turn everything into millilitres so nothing can go wrong.
5 l = 5000 ml. 2 l 400 ml = 2400 ml.
5000 − 2400 = 2600 ml.
2600 ml = 2 l 600 ml left in the jerrycan.
Tip
How to remember which way to go
Ask yourself one question: am I ending up with big things or small things?
Small things are easy to collect, so you need lots of them - the number goes UP, so you multiply.
Big things are heavy to collect, so you need few of them - the number goes DOWN, so you divide.
And notice how kind the metric system is: km to m, kg to g, and litres to ml all use the same friendly 1000.
Do these on paper first, then check. The answers are right underneath, so you are never stuck.
What you learned today
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