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GST, Budgeting and Household Expenses

Every time you buy something in Australia, 10% goes to the government as GST, but only on the pre-GST price, never on the GST-inclusive price again. Master the three GST calculations, build a budget across different time periods, and decode tiered utility bills.

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Learning Intentions

Three printable worksheets that build from foundations to mastery, or build your own from any module’s questions.

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Three printable worksheets that build from foundations to mastery, or build your own from any module’s questions.

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Think First

Every time you buy something in Australia, a coffee, a pair of shoes, a phone, 10% of that price goes straight to the government as GST. But not everything attracts GST: fresh food, medical services, and some education costs are GST-free. If you've ever wondered why a supermarket receipt shows some items with GST and others without, this lesson explains the system. And when you move out of home, the bills don't stop at groceries, electricity, gas, water, council rates, insurance. How do people track all of this and make sure they don't spend more than they earn?

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Learning Intentions

Calculate GST-inclusive prices, pre-GST prices, and the GST component from a GST-inclusive amount
Construct a budget table, calculate total income and expenses, and determine surplus or deficit
Calculate household utility bills with fixed supply charges and tiered usage rates
Convert budget figures between weekly, fortnightly, monthly and annual time periods
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Key Terms

GSTGoods and Services Tax, a 10% tax on most goods and services in Australia.
BudgetA plan that balances income against expenses over a set period.
Fixed ExpenseCosts that stay the same each period (rent, loan repayments, insurance).
Variable ExpenseCosts that change from period to period (groceries, utilities, petrol).
Discretionary SpendingNon-essential spending on wants rather than needs (entertainment, dining out).
A GST-inclusive price is $1,430. What is the pre-GST price?
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GST, Calculating In Both Directions

GST is a 10% tax added to the price of most goods and services in Australia, the key skill is moving confidently between...

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GST, Calculating In Both Directions

GST is a 10% tax added to the price of most goods and services in Australia, the key skill is moving confidently between pre-GST, GST-inclusive, and GST-component amounts.

There are three types of GST calculations you need to master:

  1. Adding GST: multiply the pre-GST price by 1.10. Example: $85 × 1.10 = $93.50.
  2. Finding the GST amount on a pre-GST price: multiply by 0.10. Example: $85 × 0.10 = $8.50.
  3. Removing GST from a GST-inclusive price: divide by 1.10. Example: $93.50 ÷ 1.10 = $85.00.

The ÷ 11 shortcut works because the GST component is 10/110 = 1/11 of the GST-inclusive price. Common GST-free items in Australia include: fresh food, most medical services, childcare, and some educational courses.

Must do: Identify whether the given price is pre-GST or GST-inclusive before calculating. Applying ×1.10 to a GST-inclusive price double-counts the tax.
Common error: To find the GST component in a GST-inclusive price, divide by 11, not multiply by 0.10. If a price of $\$110$ already includes GST, the GST component is $\$110$ ÷ 11 = $\$10$, not $\$110$ × 0.10 = $11.

Three GST directions: Pre-GST → inclusive: ×1.10. Inclusive → pre-GST: ÷1.10. GST component from inclusive price: ÷11 (because GST = 10/110 = 1/11 of inclusive price). Never multiply an inclusive price by 0.10, that overcounts the tax.

Pause, copy the three GST rules: pre-GST → inclusive: ×1.10; inclusive → pre-GST: ÷1.10; GST component from inclusive price: ÷11 (not ×0.10, that overcounts because the price already includes the GST) into your book.

GST three calculation directions as a comparison table of start value, operation and result
True or False: To find the GST component from a GST-inclusive price of $\$242$, you multiply $\$242$ by 0.10.
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Budgeting, Surplus, Deficit and Balance

We just saw the three GST directions: pre-GST → inclusive (×1.10), inclusive → pre-GST (÷1.10), and GST component from an...

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Budgeting, Surplus, Deficit and Balance

We just saw the three GST directions: pre-GST → inclusive (×1.10), inclusive → pre-GST (÷1.10), and GST component from an inclusive price (÷11, because GST = 10/110 = 1/11 of the inclusive price). That raises a question: GST is one single calculation, but in real life people need to track all income and all spending across a month or year to stay solvent. How do you test whether a budget is sustainable? This card answers it → a budget maps all income against all expenses in the same time period; surplus = income > expenses; deficit = expenses > income.

A budget is a plan that maps income against expenses over a defined time period, the goal is to ensure spending does not exceed earning.

A personal or household budget lists all income sources (wages, government payments, investment income) and all expenses (rent/mortgage, utilities, food, transport, entertainment, insurance) for a fixed period. The difference between total income and total expenses determines whether the budget is in:

  • Surplus: income > expenses, money left over
  • Deficit: expenses > income, spending more than earned
  • Balanced: income = expenses

HSC questions often require you to convert all figures to the same time period before comparing. A common extension asks how to eliminate a deficit, either by increasing income or reducing specific expense categories.

Must do: Convert all figures to the same time period before calculating surplus or deficit. A question may give weekly rent, monthly phone bills, and annual insurance, convert everything to weekly (or monthly or annual) before summing.
Common error: A budget deficit is not automatically a crisis, but it is unsustainable. A household running a weekly deficit of $\$80$ will accumulate $\$4,160$ in debt over a year.

Budget: surplus = income > expenses; deficit = expenses > income. Convert all figures to the same time period before summing. Annual surplus = monthly surplus × 12; weekly surplus = monthly surplus × 12 ÷ 52. A sustained deficit accumulates as debt.

Pause, copy the budget surplus/deficit formula (surplus = total income − total expenses when positive; deficit when negative), the time-period conversion rules (weekly × 52 = annual; monthly × 12 = annual), and the warning to convert all figures to the same period before summing into your book.

A household has monthly income of $\$4,860$ and monthly expenses of $\$5,140$. Their annual budget is $\$$.
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Household Expenses, Utilities, Rates and Bills

We just saw budget surplus and deficit, and that all figures must be converted to the same time period before comparing.

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Household Expenses, Utilities, Rates and Bills

We just saw budget surplus and deficit, and that all figures must be converted to the same time period before comparing. That raises a question: utility bills like electricity aren't a single flat amount, they have an internal structure. How do you calculate the total for a bill that charges both a fixed supply fee and a per-unit usage fee? This card answers it → utility bill total = supply charge (daily rate × days) + Tier 1 usage (first N units × rate 1) + Tier 2 usage ((total − N) units × rate 2).

Household bills follow predictable mathematical structures, unit rates, fixed charges, tiered pricing, that require careful reading before any calculation.

Common household expense calculations in HSC questions include:

  • Electricity bills: fixed supply charge per day + variable usage charge per kWh
  • Water bills: fixed service charge + usage charge per kilolitre, sometimes tiered
  • Council rates: usually a fixed annual amount calculated as a percentage of land value
  • Insurance premiums: annual or monthly, sometimes with discounts for upfront payment

Tiered pricing means the first X units are charged at one rate, and additional units at a higher rate, this is structurally identical to tiered commission from earlier lessons.

Must do: Check units on utility bills, kWh vs MJ, litres vs kilolitres. Always confirm the unit before multiplying by the rate.
Common error: Don't forget the fixed supply/service charge. Every electricity and water bill has a fixed daily or quarterly charge regardless of usage. Students routinely miss this charge, producing a total that is consistently too low.

Utility bill formula: Total = supply charge (daily rate × days) + Tier 1 usage (first N units × rate1) + Tier 2 usage ((total − N) × rate2). GST-inclusive total = pre-GST total × 1.10. Never omit the fixed daily supply charge.

Pause, copy the utility bill formula: Total = supply charge (daily rate × days) + Tier 1 usage (first N units × rate 1) + Tier 2 usage ((total used − N) × rate 2), and the final GST step if prices are given excluding GST into your book.

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Choosing the Right Operation Quickly

We just saw utility bills combining a fixed daily supply charge and tiered usage rates, the most common error is forgetting...

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Choosing the Right Operation Quickly

We just saw utility bills combining a fixed daily supply charge and tiered usage rates, the most common error is forgetting the supply charge. That raises a question: an exam question mixes GST-inclusive prices, budget time periods, and bill tiers, how do you instantly choose the right operation before calculating? This card answers it → label each amount first ("inclusive price", "monthly expense", "daily supply charge") and use the decision table to map each label to the correct first operation.

Many mistakes in this topic happen before the arithmetic starts. If you identify the type of amount first, the correct operation usually becomes obvious.

If the question says… You likely need…
"before GST" or "exclusive of GST"Multiply by 1.10 to get the GST-inclusive price
"including GST" or "GST-inclusive"Divide by 1.10 to get the pre-GST price
"GST component" from an inclusive priceDivide by 11
"weekly, monthly and annual figures mixed"Convert everything to one common time period first
"daily charge plus usage charge"Calculate both parts, then add
"first X units at one rate, remainder at another"Split the usage into tiers before multiplying
Exam technique: Write a short label beside each number before calculating, such as "inclusive price", "monthly income", or "daily supply charge". That small habit helps prevent using the wrong formula or mixing time periods.

Identify the amount type before operating: 'pre-GST' → ×1.10 to add; 'inclusive' → ÷1.10 to remove or ÷11 for component; 'weekly/monthly/annual mixed' → convert to one period first; 'fixed + usage' → calculate both parts then add. Label amounts before calculating.

Pause, copy the amount-type decision table: "pre-GST" → ×1.10; "GST-inclusive" → ÷1.10 or ÷11; "weekly/monthly/annual mixed" → convert to one period first; "fixed supply charge + usage" → calculate both parts then add into your book.

Match each calculation to the correct GST operation.
  • Pre-GST $85 → find GST-inclusive price
  • GST-inclusive $110 → find pre-GST price
  • GST-inclusive $242 → find GST component
  • Pre-GST $340 → find GST amount
  • Multiply by 0.10
  • Divide by 11
  • Divide by 1.10
  • Multiply by 1.10
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GST calculations, both directions

(a) A plumber charges $340 before GST.

WE1

GST calculations, both directions

(a) A plumber charges $340 before GST. What is the GST-inclusive price?

(b) A laptop is advertised at $1,628 including GST. What is the pre-GST price and the GST component?

Part (a), Step 1

$$\text{GST-inclusive price} = \$340 \times 1.10 = \$374.00$$

Multiply pre-GST price by 1.10 to add 10% GST in one step.

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Budget construction, surplus and annual position

The Chen family has the following monthly finances. Income: wages $\$5,840$, rental income $\$620$. Expenses: mortgage $\$2,100$, groceries $\$780$, utilities $\$310$, transport $\$420$, insurance $\$195$, entertainment $\$380$, clothing $150.

Calculate their monthly surplus or deficit and their annual position.

Step 1

$$\text{Total monthly income} = \$5{,}840 + \$620 = \$6{,}460$$

Sum all income sources for the month.

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Electricity bill, tiered pricing and GST

An electricity bill shows: supply charge $1.04 per day for 90 days; usage charge, first 500 kWh at 28.6c/kWh, remaining usage...

WE3

Electricity bill, tiered pricing and GST

An electricity bill shows: supply charge $1.04 per day for 90 days; usage charge, first 500 kWh at 28.6c/kWh, remaining usage at 34.2c/kWh. The household used 720 kWh over the 90-day period.

Calculate the total bill (excluding GST), then find the GST-inclusive total.

Step 1

$$\text{Supply charge} = \$1.04 \times 90 = \$93.60$$

Fixed daily charge × number of days. This applies regardless of electricity used.

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Budget comparison after converting time periods

A student has weekly income from casual work of $\$420$ plus a monthly allowance of $\$260$ from home.

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Budget comparison after converting time periods

A student has weekly income from casual work of $\$420$ and receives a monthly allowance of $\$260$ from home. Their expenses are: rent $\$180$ per week, groceries $\$95$ per week, phone $\$38$ per month, transport $\$42$ per week, and insurance $624 per year.

Convert everything to a weekly budget and determine whether the student has a weekly surplus or deficit.

Step 1

$$\text{Weekly allowance} = \$260 \times 12 \div 52 = \$60.00$$

Convert the monthly amount to annual, then divide by 52 to get a weekly figure.

Name THREE types of information you must identify before calculating a household electricity bill.
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