GST, Budgeting and Household Expenses

Calculate GST on goods and services, construct and analyse a personal budget, and compare household expenses including utilities and council rates.

55 min MS-F1 4 MC 4 WE Lesson 9 of 14 Free
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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?

Learning Intentions

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Calculate GST-inclusive prices, pre-GST prices, and the GST component from a GST-inclusive amount

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Construct a budget table, calculate total income and expenses, and determine surplus or deficit

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Calculate household utility bills with fixed supply charges and tiered usage rates

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Convert budget figures between weekly, fortnightly, monthly and annual time periods

Key Formulas

GST — Adding
$$\text{GST-inclusive price} = \text{Pre-GST price} \times 1.10$$
Multiply by 1.10 to add 10% GST in one step
GST — Removing
$$\text{Pre-GST price} = \text{GST-inclusive price} \div 1.10$$
Divide by 1.10 to strip GST from an inclusive price
GST Component from Inclusive Price
$$\text{GST component} = \text{GST-inclusive price} \div 11$$
Equivalent to price × 10/110; always use ÷11, not ×0.10, on inclusive prices
Budget Surplus / Deficit
$$\text{Surplus} = \text{Total income} - \text{Total expenses}$$
Positive result = surplus; negative result = deficit (expenses exceed income)

Three GST Calculations at a Glance

Pre-GST Price e.g. $340.00 (exclusive of GST) GST-Inclusive Price e.g. $374.00 (includes 10% GST) GST Component e.g. $34.00 × 1.10 ÷ 1.10 ÷ 11 × 0.10 GST: Three Calculation Directions Orange arrows: convert between pre-GST and GST-inclusive | Green/blue: find the GST component

Core Concepts

Misconceptions to Fix

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Wrong: GST is calculated on the discounted price when a sale is on.

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Right: GST is calculated on the pre-discount price unless the advertised price is GST-inclusive. A $100 item with 10% discount and 10% GST: GST is $10 (on $100), not $9 (on $90).

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. HSC questions will always specify whether a price is GST-inclusive or exclusive.

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. Read the question carefully — "price before GST" and "price including GST" require completely different operations.
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. Multiplying an inclusive price by 0.10 overcounts the GST.

Budgeting — Surplus, Deficit and Balance

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 — usually weekly, fortnightly, or monthly. 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. HSC questions sometimes ask how long until savings are exhausted or debt reaches a threshold — recognise these as rate × time problems.
Insight: The 50/30/20 budgeting guideline (a financial literacy framework, not an HSC formula): 50% of after-tax income on needs, 30% on wants, 20% on savings. Useful context for budget analysis questions that ask you to comment on the reasonableness of a spending plan.

Household Expenses — Utilities, Rates and Bills

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 Lesson 3. Comparing two providers requires calculating the total cost under each plan for the same usage, then finding the difference.

Must do: Check units on utility bills — kWh vs MJ, litres vs kilolitres. Electricity is measured in kilowatt-hours (kWh); gas is sometimes in megajoules (MJ). Water is billed in kilolitres (kL = 1,000 litres). 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 calculate only the usage component and miss the supply charge, producing a total that is consistently too low.

Choosing the Right Operation Quickly

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 together"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.

Worked Examples

Example 1

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.

Example 2

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.

Example 3

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 how much electricity was used.

Example 4

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 an annual amount first, then divide by 52 to get a weekly figure.

Revisit Your Initial Thinking

Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?

Checkpoint — Test Yourself

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).
MC

Multiple Choice

5 random questions from a replayable lesson bank — feedback shown immediately

B $1,300.00
C $143.00
D $1,573.00

A household has monthly income of $4,860 and monthly expenses of $5,140. What is their annual budget position?

A $280 annual surplus
B $3,080 annual surplus
C $280 annual deficit
D $3,360 annual deficit

A water bill has a fixed service charge of $84 per quarter and a usage charge of $2.40 per kL. If a household uses 37 kL in the quarter, what is the total bill before GST?

A $84.00
B $88.80
C $172.80
D $190.08

Which method correctly finds the GST component from a GST-inclusive price of $242?

A $242 ÷ 11
B $242 × 0.10
C $242 ÷ 10
D $242 - 11

Written Response Practice

These questions focus on clear setup: identify the type of price or bill first, then show each line of working in a consistent time period.

Short Answer 1

A tradesperson quotes $1,760 including GST for a job. Calculate the pre-GST price and the GST component.

Short Answer 2

A household has fortnightly income of $2,240. Their fortnightly expenses are rent $820, groceries $310, transport $145, and phone $96 per month. Determine whether they have a fortnightly surplus or deficit.

Short Answer 3

An electricity bill has a supply charge of $0.98 per day for 91 days and a usage charge of 31.5c per kWh. The household used 540 kWh. Calculate the total bill before GST and the GST-inclusive total.

Budget and Bill Builder

Use this as a fast consolidation drill: identify the structure first, then choose the correct operation.

To move from a pre-GST price to a GST-inclusive price, what should you do?

A Divide by 1.10
B Divide by 11
C Multiply by 1.10
D Multiply by 0.10

A budget uses weekly rent, monthly subscriptions and annual insurance. What should you do before adding the expenses?

A Round each value to the nearest dollar
B Convert all figures to the same time period
C Add only the weekly amounts
D Multiply everything by 12

A bill includes a fixed daily charge and a usage charge. Which result would definitely be too small?

A Adding both charges together
B Calculating usage first, then fixed charge
C Finding the usage part and then including GST
D Using only the usage charge

If expenses are greater than income in a budget, the result is called a:

A Deficit
B Surplus
C Rebate
D Discount
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