Measurement and Money
Primary 5 · Mathematics
Measurement and Money
Lesson 3 of 3
13 min
Let's begin
Every morning Uwase carries a basin of avocados to the market. She pays the farmer, she sells to her customers, she counts out change, and at sunset she knows exactly whether the day was good.
She is not doing anything magic. She is doing the arithmetic in this lesson - carefully, and in Rwandan francs.
By the end of today, you can keep books like Uwase.
Worked example
One exercise book costs 350 Frw. Mugisha buys 4 of them. What does he pay?
Four books, each 350 Frw. That is four equal groups, so multiply.
350 × 4 = 1,400 Frw.
Worked example
(going backwards). A shopkeeper sells 6 identical pens for 1,800 Frw altogether. What does one pen cost?
Here the total is shared out into 6 equal parts, so divide.
1,800 ÷ 6 = 300 Frw for one pen.
Worked example
Keza buys bread for 800 Frw and a packet of milk for 450 Frw. She hands the shopkeeper a 2,000 Frw note. How much change should she get back?
Never rush to subtract. Find the total cost first.
800 + 450 = 1,250 Frw.
Now take that away from the money she gave.
2,000 - 1,250 = 750 Frw change.
If the shopkeeper hands her only 700 Frw, Keza knows to speak up. That is the power of checking.
Change = money you hand over - total cost.
A trader has two prices in her head at all times:
Worked example
(a good day). Habimana buys a sack of potatoes for 12,000 Frw. He sells the whole sack for 15,500 Frw.
He got back more than he put in, so he has made a profit.
Profit = selling price - buying price = 15,500 - 12,000 = 3,500 Frw profit.
Worked example
(a hard day). Umutoni buys tomatoes for 8,000 Frw. Heavy rain spoils some of them, and she only manages to sell the rest for 6,200 Frw.
She got back less than she put in, so she has made a loss.
Loss = buying price - selling price = 8,000 - 6,200 = 1,800 Frw loss.
Notice that you always subtract the smaller amount from the bigger one. The name of the answer - profit or loss - depends on which price was bigger.
Gatete buys 20 mangoes at 150 Frw each and sells every one of them at 200 Frw.
Buying price for all of them: 20 × 150 = 3,000 Frw.
Selling price for all of them: 20 × 200 = 4,000 Frw.
Selling price is bigger, so it is a profit: 4,000 - 3,000 = 1,000 Frw profit.
The quick way: he gains 200 − 150 = 50 Frw on each mango, and 50 × 20 = 1,000 Frw. Same answer, and it proves you understood it.
A budget is simply a plan written down before you spend, not a sad story you tell afterwards.
Worked example
Ishimwe receives 5,000 Frw each month. He plans: 1,500 Frw for transport, 1,200 Frw for snacks, 800 Frw for exercise books. Whatever is left, he saves.
Total spending: 1,500 + 1,200 + 800 = 3,500 Frw.
Savings: 5,000 - 3,500 = 1,500 Frw every month.
He wants a football that costs 5,500 Frw. How many months must he wait?
After 1 month: 1,500. After 2: 3,000. After 3: 4,500 - still not enough. After 4 months: 6,000 Frw.
So Ishimwe buys his football in the 4th month, and he still has 6,000 − 5,500 = 500 Frw left over.
Money you do not spend is money that waits for you.
Tip
The whole lesson in four short lines
And one habit that saves you every time: work out the total first, subtract second.
Work them out on paper, then check underneath.
What you learned today
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