Budgeting with percentages and fixed values

A budget lists weekly income, monthly internet and quarterly electricity. What must happen before you subtract these costs from income?

Predict: weekly income is $600. Essential costs are $340, transport $60 and flexible spending $100. If flexible spending rises by $40 while everything else stays fixed, what happens to the remaining balance?

Move flexible spending. Compare $100, $200 and $240. The table includes all costs in this simplified weekly model; negative remaining balance means spending exceeds income

What happens when flexible spending passes $200?

What does “left over” mean?

Net income is money available after deductions such as tax. A budget is a plan for that income. Fixed costs are treated as unchanged within the stated period; variable costs are estimates that can change. These examples are fictional, not recommended spending allocations

1. 1Remaining balance For income I and total spending E in the same period, R is the balance. It is available for savings only after all relevant spending has been included \(\displaystyle R=I-E\)
2. 2Include all spending If the table lists only recurring bills, its remaining balance must also cover food, irregular costs and anything else omitted before it can be treated as savings
3. 3Read the sign A positive balance is money remaining; zero is balanced; a negative balance is a deficit

Annual net income

\(\displaystyle \$42{,}000\)

Transport share

\(\displaystyle 18\%\)

Find the annual allocation for transport

Convert 18% to 0.18, then multiply by the annual net income

A complete annual budget assigns 35% of income to housing, 22% to food, 13% to transport and 10% to all other expenses. The rest is savings. What percentage is available for savings?

Annual net income

\(\displaystyle \$48{,}000\)

Expense share

\(\displaystyle 78\%\)

All annual expenses together use 78% of net income. The rest is allocated to savings. Find the annual savings amount

First subtract 78 from 100 to find the savings percentage, then apply that share to income

Put every payment on the same clock

For this lesson, assume payments repeat unchanged and use 52 weeks, 26 fortnights, 12 months and 4 quarters in one model year. Actual payment dates and calendar years can differ

1. 1Annual cost A payment c repeated k times in the model year has annual cost C. Multiply the payment by the count \(\displaystyle C=kc\)
2. 2Average monthly cost M is the monthly average of annual cost C. Divide by 12; this is a planning allocation, not the actual amount paid in each calendar month \(\displaystyle M=C/12\)
3. 3Avoid four-week months Four weeks per month would account for only 48 weeks per year. Use the stated yearly conversion model instead

Payment per fortnight

\(\displaystyle \$185\)

Fortnights per model year

\(\displaystyle 26\)

Find the annual cost under the 26-fortnight model. The payment stays unchanged

Multiply the fortnightly payment by 26; multiplying by 12 would treat it as a monthly payment

Internet per month

\(\displaystyle \$75\)

Electricity per quarter

\(\displaystyle \$310\)

Insurance per year

\(\displaystyle \$640\)

Find the annual total of these three listed costs. Use 12 months and 4 quarters per model year; payments do not change

Annualise each row separately: monthly × 12, quarterly × 4, yearly × 1. Then add

Annual listed costs

\(\displaystyle \$18{,}960\)

Months per model year

\(\displaystyle 12\)

Find the average monthly allocation for these annual costs. This is an average, not the actual bill in each month

Divide the annual cost total by 12 months

Annual net income

\(\displaystyle \$36{,}400\)

Annual listed costs

\(\displaystyle \$26{,}000\)

Find the average weekly balance after the listed costs, using 52 weeks per model year. Other spending is not included, so do not automatically call this savings

Subtract annual listed costs from annual income, then divide the balance by 52

Annual cost

\(\displaystyle \$7{,}540\)

Annual net income

\(\displaystyle \$52{,}000\)

What percentage of annual net income is used by this annual cost? Give one decimal place

Divide the annual cost by annual net income, multiply by 100, and give exactly one decimal place

An annual budget allocates $4,800 to transport. A monthly summary lists transport as $4,800 per month. What correction makes the monthly average consistent with the annual plan?

Annual net income

\(\displaystyle \$30{,}000\)

All annual spending

\(\displaystyle \$31{,}500\)

Find the signed annual balance. A negative answer means a deficit. All spending is included in the total

Subtract spending from income in that order. Keep the negative sign if spending is larger

Weekly net income

\(\displaystyle \$600\)

Weekly essential costs

\(\displaystyle \$340\)

Weekly transport

\(\displaystyle \$60\)

Minimum weekly savings

\(\displaystyle \$80\)

Find the largest weekly flexible-spending amount that still leaves at least $80 for savings. In this simplified model all spending is listed and the amounts remain fixed

Reserve essential costs, transport and the savings goal first. The remaining income is the maximum flexible-spending allowance

A student models next year using last month’s income and bills, multiplied by 12. Which check best improves the budget?

Make and defend a small budget

Independent task: use fictional weekly net income of $720, essential costs $380, transport $70, flexible spending $140 and a savings goal of $100. Decide whether the plan meets the goal. Then increase essential costs by 10%, revise flexible spending to keep the goal, and explain what you changed. Show a check that all allocations total the income

1. 1State assumptions Say what costs are included, which may vary and whether income is stable
2. 2Evaluate and report Explain whether your new plan is realistic and what missing information would change it