What you'll learn
Consumer arithmetic is the mathematics of buying and selling: working out a total bill, giving change, comparing which packet is better value, and calculating profit, loss and simple wages. Every calculation uses money written to two decimal places, so $4.50 means four dollars and fifty cents. This is one of the most practical topics on the SEA Mathematics paper and one of the easiest to practise, because the situations happen in every shop and market in Trinidad and Tobago. Questions are usually set in two or three steps, so the arithmetic is rarely hard — the marks are lost by answering the wrong step.
Key terms and definitions
Total — the sum of all the amounts, before anything is taken away.
Change — the money handed back after paying, found by subtracting the total from the amount given.
Unit price (unit cost) — the cost of one item, or the cost per kilogram or per litre. Found by dividing the price by the quantity.
Best buy — the option with the lowest unit price, which is not always the cheapest packet.
Cost price — what the seller paid for an item.
Selling price — what the seller charges the customer.
Profit — selling price minus cost price, when the selling price is higher.
Loss — cost price minus selling price, when the selling price is lower.
Discount — an amount taken off the marked price, often given as a percentage.
Wage — money earned for work, usually calculated as a rate per hour multiplied by the hours worked.
Core concepts
Writing and reading money
Money is written with a dollar sign and exactly two decimal places: $4.50, $12.05, $100.00. The two places are hundredths of a dollar, which is what a cent is.
The zero matters. Nine dollars and five cents is $9.05, not $9.5 — writing $9.5 would mean nine dollars and fifty cents. When adding or subtracting, line up the decimal points, not the last digits.
Totals and change
Adding a bill is ordinary decimal addition. Add the cents first and carry any whole dollars into the dollars column:
$4.75 + $6.50 → 75 + 50 = 125 cents, which is $1.25, so the total is $10 + $1.25 = $11.25.
For change, subtract the total from the amount handed over. Counting on is usually safer than column subtraction: from $13.85 to $14.00 is 15 cents, and from $14.00 to $20.00 is $6, so the change is $6.15.
The most common error in the whole topic is answering with the total when the question asked for the change, or the other way round. Underline which one is being asked for.
Unit price and best buy
To compare two packets of different sizes, work out what one unit costs in each:
- 2 kg for $24 → $24 ÷ 2 = $12 per kilogram
- 3 kg for $33 → $33 ÷ 3 = $11 per kilogram
The 3 kg packet costs more in total but less per kilogram, so it is the better buy. Comparing the packet prices alone would give the wrong answer, which is exactly what these questions are designed to test.
The same method works for drinks by the litre, cloth by the metre and anything sold by quantity.
Profit and loss
Profit and loss compare what the seller paid with what the seller charged.
- Cost price $60, selling price $75 → profit of $15
- Cost price $60, selling price $52 → loss of $8
If a question gives the profit and asks for the selling price, add. If it gives the selling price and the profit and asks for the cost price, subtract. Writing down which two of the three quantities you have been given makes the direction obvious.
Discounts
A discount is an amount taken off. When it is given as a percentage, find the percentage first and then subtract it.
A jacket costs $400 with 20% off: 10% of $400 is $40, so 20% is $80. The discount is $80 and the new price is $400 − $80 = $320.
Both numbers are correct answers to different questions. Read carefully whether the question wants the amount taken off or the price now paid.
Wages
A wage is usually a rate multiplied by a time: $60 an hour for 8 hours is 8 × $60 = $480. For a working week, multiply by the number of days: 5 days of 8 hours at $60 is 40 × $60 = $2 400.
Watch for part-hours. Half an hour at $60 an hour is $30, not $60.
Worked examples
Example 1: Total, then change
Devon buys 3 roti at $18.50 each and pays with a $100 note. How much change does he receive?
First the total: 3 × $18.50. Work it as 3 × $18 = $54 and 3 × 50c = $1.50, giving $55.50.
Then the change: $100.00 − $55.50 = $44.50.
The answer $55.50 is what he spent, not what he is owed. Two-step money questions almost always offer the first step as a wrong option.
Example 2: Best buy
A shop sells rice as 5 kg for $65 or 4 kg for $52. Which is the better buy?
Unit price of the 5 kg bag: $65 ÷ 5 = $13 per kilogram. Unit price of the 4 kg bag: $52 ÷ 4 = $13 per kilogram.
They are exactly the same value, at $13 per kilogram. The 4 kg bag is cheaper to buy but not better value, and neither is the "best buy" — a perfectly fair answer that children often distrust because they expect one to win.
Example 3: Discount then a voucher
A television costs $2 400. The shop takes 30% off, and Shanice also has a $150 voucher off the sale price. How much does she pay?
10% of $2 400 is $240, so 30% is 3 × $240 = $720.
The sale price is $2 400 − $720 = $1 680.
Then the voucher: $1 680 − $150 = $1 530.
Taking the voucher off first and then the percentage would give a different answer, so the order stated in the question has to be followed exactly.
Common mistakes and how to avoid them
Giving the total when the change was asked for. Fix: underline the final question before calculating anything.
Writing money with one decimal place. Recording nine dollars and five cents as $9.5. Fix: money always has two places after the point.
Lining up the last digits instead of the decimal points. Adding $12.50 and $7.05 by lining up the 0 and the 5. Fix: the decimal points go one under the other, always.
Choosing the cheaper packet as the best buy. Fix: best value means lowest cost per unit, so divide before comparing.
Giving the discount when the sale price was asked for. A jacket reduced by $80 does not cost $80. Fix: check whether the question says "taken off" or "now costs".
Forgetting to multiply before subtracting. Buying 3 items at $2.35 and subtracting only one from $10. Fix: total first, then subtract.
How parents can help at home
This is the topic where everyday life does the teaching for you, and where a parent can help most without knowing any special method.
Hand over the shopping. Let your child add the items before you reach the till, then predict the change. Being right about real money is far more motivating than a tick on a worksheet.
Ask the best-buy question in the aisle. "This one is 2 kg for $24 and this one is 3 kg for $33 — which should we take?" The answer requires dividing, and the reason for dividing is obvious when a real basket is involved.
Talk about a small sale. If your child sells anything at a school fair, work out the cost price and the selling price together afterwards. Profit and loss stop being abstract the moment the money is theirs.
Check the two-step habit. When your child gives an answer, ask "is that what they spent or what they got back?" Most lost marks in this topic are answers to the wrong question rather than arithmetic errors, and this one prompt catches nearly all of them.
Exam technique for money and consumer arithmetic
Write the dollar sign and two decimal places in every answer. An answer of "44.5" is ambiguous where "$44.50" is not.
Set out multi-step questions in labelled lines — total, then change; or discount, then sale price. It costs a few seconds and makes it obvious which number answers the question.
Estimate before calculating. Three roti at about $20 each is about $60, so change from $100 should be around $40. An answer of $4.45 or $445 can then be spotted immediately.
Look at the answer options in multiple-choice items. If one option is the total and another is the change, the question is testing whether you read it properly, not whether you can subtract.
Quick revision summary
- Money is written with two decimal places: $9.05, not $9.5.
- Line up decimal points when adding or subtracting, never the last digits.
- Change = money handed over − total cost.
- Unit price = price ÷ quantity; the best buy has the lowest unit price, not the lowest total price.
- Profit = selling price − cost price. Loss = cost price − selling price.
- For a percentage discount, find the percentage first, then subtract it from the marked price.
- The discount and the sale price are different answers — check which is asked for.
- Wage = rate per hour × hours worked; half an hour earns half the rate.
- In two-step questions, the first step is almost always offered as a wrong option.