The idea, step by step
Add every required cost
A budget is an amount available for a purpose. List all required costs using the same units and quantities, add them, then compare the total with the money available. Money left is the budget minus total spending.
A cheap headline price can be misleading if delivery, equipment or another compulsory fee is omitted. State whether an item is a need for the plan or an optional extra.
| Item | Cost in dollars |
|---|---|
| Sandwiches | 24 |
| Drinks | 16 |
| Fruit | 12 |
Compare a fair amount
Offers with different pack sizes cannot be compared using the pack price alone. Scale them to a common quantity, or find the cost of one item if helpful. Also check that the cheaper option supplies enough items.
The best decision depends on the question. Lowest total spending, lowest cost per item, and least waste may select different offers. Explain the criterion and any assumptions, rather than writing only a shop name.
A common trap
The smallest price on the label always gives the best value.
Instead: Compare the same quantity and include all required costs. A lower pack price may buy fewer items.
See the thinking
Check a picnic plan
A picnic costs 24 dollars for sandwiches, 16 dollars for drinks and 12 dollars for fruit. The budget is 60 dollars. How much remains? Can two extra fruit packs costing 4 dollars each be added?
| Item | Cost in dollars |
|---|---|
| Sandwiches | 24 |
| Drinks | 16 |
| Fruit | 12 |
- Total = 24 + 16 + 12 = 52 dollars.
- Money left = 60 - 52 = 8 dollars.
- Two extra packs cost 8 dollars, so the revised plan uses exactly 60 dollars.
Compare a common quantity
A pack of 4 pens costs 3.60 dollars. A pack of 6 pens costs 4.80 dollars. Which option is cheaper when buying exactly 12 pens?
- Three 4-packs cost 3 times 3.60 = 10.80 dollars.
- Two 6-packs cost 2 times 4.80 = 9.60 dollars.
- The 6-pack option saves 10.80 - 9.60 = 1.20 dollars.
Check your understanding
A 35-dollar budget pays for items costing 12.80 and 9.60 dollars. What remains?
Show answer
12.60 dollars
Total spending is 22.40 dollars; 35.00 - 22.40 = 12.60.
Your turn
6 questions · 16 marks · Work at your own pace.
Read the questions online
- A group has 80 dollars. Food costs 32.50 dollars and transport costs 28 dollars. Find the money left.2 marks
- Buy exactly 10 pencils: packs of 2 cost 1.40 dollars, or packs of 5 cost 3 dollars. Which option costs less?3 marks
- You spend of a 50-dollar budget on food and another 18 dollars on travel. What remains?3 marks
- A game costs 26 dollars online plus 6 dollars delivery, or 31 dollars in a local shop. Which total is lower?2 marks
- Exactly 12 people need transport. Option A costs a flat 60 dollars; option B costs 6 dollars per person. Both have enough seats. Compare costs.3 marks
- A class has 100 dollars. It needs 8 art kits at 9.50 dollars each and a 12-dollar delivery. Can it also buy a 15-dollar poster?3 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- A group has 80 dollars. Food costs 32.50 dollars and transport costs 28 dollars. Find the money left.2 marks
Total spending is 60.50 dollars; subtract from 80.
19.50 dollars - Buy exactly 10 pencils: packs of 2 cost 1.40 dollars, or packs of 5 cost 3 dollars. Which option costs less?3 marks
Five 2-packs cost 7 dollars; two 5-packs cost 6 dollars.
Two 5-packs cost 6 dollars, saving 1 dollar. - You spend of a 50-dollar budget on food and another 18 dollars on travel. What remains?3 marks
Food costs 15 dollars; total spending is 33 dollars.
50 - 33 = 17 dollars.
17 dollars - A game costs 26 dollars online plus 6 dollars delivery, or 31 dollars in a local shop. Which total is lower?2 marks
Online total = 26 + 6 = 32 dollars; local total = 31 dollars.
The local shop, by 1 dollar. - Exactly 12 people need transport. Option A costs a flat 60 dollars; option B costs 6 dollars per person. Both have enough seats. Compare costs.3 marks
Option B: 12 times 6 = 72.
72 - 60 = 12 dollars.
A costs 60 dollars and B costs 72 dollars; A saves 12 dollars. - A class has 100 dollars. It needs 8 art kits at 9.50 dollars each and a 12-dollar delivery. Can it also buy a 15-dollar poster?3 marks
Kits and delivery cost 8 times 9.50 + 12 = 88 dollars.
Adding the poster gives 103 dollars.
No; it would exceed the budget by 3 dollars.
Curriculum reference & lesson scope
Australian Curriculum v9 / SA Curriculum Mathematics
Fractions–decimals–percentages; percentages of quantities; expressing one amount as a percentage of another; discounts and sale prices; practical spending/budget comparisons; choosing and explaining a financial model.
Official curriculum source ↗