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F1.4 · Modify a budget when circumstances change

Learn to modify a budget when circumstances change through clear examples and targeted practice.

Ontario Grade 9 Mathematics

Financial Literacy

MTH1W study topic: F1.4 — Modify a budget when circumstances change

A budget is a plan for money coming in and money going out over a set time, such as one month. Plans can change. Someone might earn less, pay a higher bill, or have a new cost. Modifying a budget means updating the plan to reflect the change. You do not need to start over: identify what changed, update the affected amount, and check the new balance.

What you will learn

  • Explain how a change in income or spending affects a budget.
  • Update budget amounts while keeping the same time period.
  • Check whether the revised budget has money left over or is short.

1. Read the budget before changing it

Imagine that you plan to spend part of your monthly allowance on a phone plan, snacks, and savings. The plan is useful only if the amounts match what is actually happening. If the phone plan becomes more expensive, the old plan no longer shows your real situation.
Income is money received during the budget period. Expenses are amounts paid or set aside for costs during that period. Savings means money you plan to keep for later. In a budget, savings can be included as one of the planned amounts going out, even though it is not a bill.
A budget balance tells you what remains after the planned expenses and savings are taken from income. A positive balance means some income is still unassigned. A balance of zero means the whole amount has been planned. A negative balance means the plan uses more money than is available, so it needs another change.
Before changing amounts, check that they cover the same period. For example, do not compare weekly income with monthly expenses without first putting them on a matching time basis. Also check whether an amount is income, an expense, or savings. This prevents changing the wrong part of the plan.
balance=income−expenses−savings\text{balance}=\text{income}-\text{expenses}-\text{savings}
  • Income is money received; expenses are money used for costs.
  • A budget must compare amounts for the same time period.
  • A negative balance means the current plan does not fit the available income.

2. Identify what changed

When circumstances change, first describe the change in plain language. For instance: “My monthly income dropped by CAD 20,” or “My transit cost rose by CAD 12.” Then decide which budget line is affected. A budget line is one named amount in the plan, such as transit, food, or income.
Next, update that amount. If income drops, use the smaller income amount. If a cost rises, use the larger expense amount. If a cost disappears, remove it or set it to zero. Keep the other amounts the same unless the situation also changes them.
Then recalculate the balance. This shows the effect of the change on the whole plan. A change of CAD 10 in an expense makes the balance CAD 10 smaller, if everything else stays the same. A change of CAD 10 in income makes the balance CAD 10 larger.
If the new balance is negative, the budget needs to be adjusted again. You might reduce a planned expense or savings amount, if that choice is possible. Do not quietly change a bill that must still be paid. Make the plan realistic, and clearly note which amount you changed.
new balance=new income−new expenses−new savings\text{new balance}=\text{new income}-\text{new expenses}-\text{new savings}
  • Name the change and locate the affected budget line.
  • Change only the amounts affected by the new circumstance.
  • Recalculate and decide whether the revised plan is realistic.

3. Use a table to keep the plan clear

A small table can make a change easy to track. Write the old amount, the change, and the new amount. Use words such as “up” or “down” so it is clear whether the amount increased or decreased. Then total the new expenses and savings and compare them with the new income.
For example, if a monthly cost rises from CAD 25 to CAD 31, the updated line is CAD 31. The increase is CAD 6. If income and every other amount stay the same, the balance becomes CAD 6 smaller. Updating the line and recalculating is safer than trying to remember the effect in your head.
Symbols can represent amounts when a budget includes several numbers. Let II mean income, EE mean total expenses, and SS mean savings. These letters are short names for the amounts, not new kinds of money. The balance is what remains after subtracting expenses and savings from income.
For a change in one amount, use the updated value in the budget. If an expense changes from EE to E+6E+6, for example, that expense is now six dollars higher. The updated balance must use this new expense amount. The table and the calculation should agree.
B=I−E−SB=I-E-S
  • Record the old amount and new amount so the change is visible.
  • Recalculate totals after updating a budget line.
  • Use symbols only as short names for budget amounts.

4. Check the revised budget

After recalculating, read the result in context. A number alone is not enough. State whether the budget has money left, balances exactly, or is short. Include the time period and currency when you explain the result.
Check your arithmetic by adding the planned expenses and savings, then comparing that total with income. If planned spending and savings are greater than income, the balance must be negative. If they are less, the balance must be positive. This check can reveal a sign or addition error.
Finally, check that the revised amounts make sense. Did you update the right line? Did you keep monthly amounts monthly? Did you count a changed cost twice? A correct calculation based on the wrong amount still gives an unhelpful budget.
planned total=expenses+savings\text{planned total}=\text{expenses}+\text{savings}
  • Explain what the balance means, not just its numerical value.
  • Use addition to check the total planned amount.
  • Confirm that each changed amount is counted once and matches the budget period.

Mina's monthly budget after the change

Budget lineBeforeChangeAfter
IncomeCAD 180No changeCAD 180
TransitCAD 55Up CAD 12CAD 67
FoodCAD 70No changeCAD 70
SavingsCAD 30No changeCAD 30
BalanceCAD 25Down CAD 12CAD 13

Worked example

A higher monthly transit cost

Mina receives CAD 180 each month. Her budget has CAD 55 for transit, CAD 70 for food, and CAD 30 for savings. Her monthly transit cost rises by CAD 12. If the other amounts stay the same, what is the revised balance, and does the budget fit her income?
  1. Update the changed amount
    Transit is the affected expense. Add the increase to its old amount to find the new monthly transit cost.
    55+12=6755+12=67
  2. Find the revised planned total
    Add the new transit cost, food amount, and savings. This gives the full amount Mina plans to use from her monthly income.
    67+70+30=16767+70+30=167
  3. Compare with income
    Subtract the planned total from income. The positive result is the amount left after the listed expenses and savings.
    180−167=13180-167=13
Answer: Mina's revised transit cost is CAD 67. Her planned expenses and savings total CAD 167, so her revised balance is CAD 13. The budget fits her monthly income, with CAD 13 left unassigned.
Check: The original planned total was CAD 155. A CAD 12 increase in transit makes it CAD 167, which is still less than the CAD 180 income.

Common mistakes and how to avoid them

Adding an expense increase to income.
Correction: An expense increase raises the planned total and lowers the balance. Update the expense line, then recalculate.
Changing every budget amount when only one circumstance changed.
Correction: Update the affected line first. Keep other amounts unchanged unless the new situation affects them too.
Comparing a weekly amount with a monthly amount directly.
Correction: Put amounts on the same time basis before calculating a budget balance.
Treating a negative balance as money left over.
Correction: A negative balance means planned expenses and savings are greater than income. The plan needs a realistic adjustment.

Lesson summary

  • A budget records income, expenses, and planned savings for a set period.
  • When circumstances change, identify and update the affected budget line.
  • Recalculate the planned total and balance using the updated amounts.
  • Explain whether the budget has money left, balances, or is short, and check that the plan is realistic.

Check your understanding

Question 1

A monthly budget has CAD 240 income and CAD 190 in expenses and savings. An expense rises by CAD 15. What is the new balance if everything else stays the same?
  1. CAD 65
  2. CAD 35
  3. CAD 50
  4. CAD 15
Show answer and explanation
CAD 35
The planned total rises from CAD 190 to CAD 205. Subtracting CAD 205 from CAD 240 leaves CAD 35.

Question 2

A person has CAD 300 monthly income and plans CAD 275 for expenses and savings. Their income drops by CAD 40. What does the revised balance show?
  1. CAD 65 left
  2. CAD 25 left
  3. A shortfall of CAD 15
  4. A shortfall of CAD 40
Show answer and explanation
A shortfall of CAD 15
The revised income is CAD 260. The planned total remains CAD 275, which is CAD 15 more than the income.

Question 3

A cost changes from CAD 48 to CAD 42. What should you do first when updating the budget?
  1. Change the income by CAD 6
  2. Replace the cost line with CAD 42
  3. Add CAD 48 and CAD 42 to the expenses
  4. Leave the cost line unchanged
Show answer and explanation
Replace the cost line with CAD 42
The cost itself is now CAD 42. Replace the old amount on that budget line, then recalculate the totals and balance.

Key terms

Budget
A plan for money received and money used or saved over a set period.
Income
Money received during the budget period.
Expense
Money paid or planned for a cost.
Balance
The amount left after expenses and savings are taken from income.
Budget line
One named amount in a budget, such as transit or food.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 9 Mathematics (MTH1W), expectation F1.4. It is a study resource, not an official curriculum publication.

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