Quick questions on Budgeting and household costs for HSC Maths Standard 2

The one idea that makes tiered bills click is that a higher rate never applies to your whole usage, only to the part inside the higher band. Picture the rate as a staircase: it is flat across the first band of usage, then steps up for the next band, and steps up again beyond that. The chart below shows a stepped electricity tariff: $25.3$ c/kWh for the first $1000$ kWh, $31.9$ c/kWh for usage between $1000$ and $2000$ kWh, then $38.5$ c/kWh above $2000$ kWh.

Owning a car has two kinds of cost. One-off (drive-away) costs are paid once to get the car on the road: the purchase price, stamp duty, and transfer or registration fees. Running costs are paid every year to keep it on the road: registration renewal and compulsory third-party (CTP) insurance, comprehensive insurance, fuel, servicing, and tyre wear.

A budget lists income on one side and expenses on the other for a chosen period, and balances to the difference. If income is larger you have a surplus; if expenses are larger you have a shortfall. The bar chart below compares one month of income with the stacked expense categories; the surplus is the gap between the two.