The idea, step by step
List the costs before deciding
A budget is a plan for money coming in and money going out. List the quantity and price of each item, then find every line total. Add all the costs before comparing them with the available money.
Keep a small amount unspent when a price is uncertain. This buffer is not a guaranteed extra cost: it is money kept available if the final costs rise. Decide what the plan includes so an important fee is not missed.
Update a prediction when something changes
If a price rises, change the affected line rather than starting with a vague new guess. Six items each rising by 50 cents add 3 dollars to the total. A fixed fee behaves differently: it is paid once, not once per item.
For fundraising, money collected is income and money spent is cost. Income minus cost is the surplus. This differs from the unused part of a spending budget, even though both calculations use subtraction.
A common trap
Comparing the price of each item with the whole budget without adding the costs.
Instead: Several affordable items can cost too much together. Add the complete planned cost first.
See the thinking
Worked example 1
A class has a picnic budget of 120 dollars. Its planned costs are shown. Find the total and the money left.
| Item | Cost in dollars |
|---|---|
| Snacks | 18 |
| Fruit | 24 |
| Drinks | 30 |
| Transport | 35 |
- Add 18+24+30+35=107 dollars.
- Compare with the budget: 120−107=13 dollars.
- The plan fits and leaves 13 dollars for changes or a buffer.
Worked example 2
A school stall collects 96 dollars. Materials cost 38.50 dollars and printing costs 12 dollars. Find its surplus.
- Total cost is 38.50+12=50.50 dollars.
- Subtract cost from income: 96−50.50=45.50 dollars.
Check your understanding
A budget is 50 dollars. Planned costs total 44 dollars. How much is left?
Show answer
6 dollars.
50−44=6. This is the unused part of the budget.
Your turn
6 questions · 11 marks · Work at your own pace.
Read the questions online
- A group has 100 dollars. It plans to spend 28, 35 and 16 dollars. Find the total and the amount left.2 marks
- Ten tickets cost 3.50 dollars each. Find their total cost.1 mark
- A fundraiser collects 84 dollars. Materials cost 31.60 dollars and printing costs 12.40 dollars. Find the surplus.2 marks
- A display needs eight packs at 2.75 dollars each and a stand costing 12.50 dollars. Find the total cost.2 marks
- Six lunches were planned at 3 dollars each. Their price rises to 3.50 dollars each. How much must be added to the budget?2 marks
- A class has a 10-dollar buffer. An extra delivery fee is 7 dollars. Explain whether the fee can be paid without removing a planned item.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- A group has 100 dollars. It plans to spend 28, 35 and 16 dollars. Find the total and the amount left.2 marks
28+35+16=79; 100−79=21.
Total 79 dollars; 21 dollars left. - Ten tickets cost 3.50 dollars each. Find their total cost.1 mark
3.50×10=35.
35 dollars. - A fundraiser collects 84 dollars. Materials cost 31.60 dollars and printing costs 12.40 dollars. Find the surplus.2 marks
Costs are 31.60+12.40=44; 84−44=40.
40 dollars. - A display needs eight packs at 2.75 dollars each and a stand costing 12.50 dollars. Find the total cost.2 marks
8×2.75=22; 22+12.50=34.50.
34.50 dollars. - Six lunches were planned at 3 dollars each. Their price rises to 3.50 dollars each. How much must be added to the budget?2 marks
Each rises by 0.50 dollars; 6×0.50=3.
3 dollars. - A class has a 10-dollar buffer. An extra delivery fee is 7 dollars. Explain whether the fee can be paid without removing a planned item.2 marks
10−7=3, provided all other planned costs stay unchanged.
Yes; 3 dollars of the buffer remains.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Fraction/decimal/percentage quantities; discounts; budgets/cost predictions; comparing choices, risks and consequences.
Official curriculum source ↗