F. Financial Literacy · Money and Finances: Paying, Saving and Smart Choices

Mental Math: Total Cost and Change

F1.2 A1About 40 minutes

Learning goals

I can add several whole-dollar prices in my head.
I can use friendly numbers and make tens to add quickly.
I can find the change from $20, $50 or $100 by counting up.
I can give change using the fewest bills and coins.
I can check my change by adding it to the total.

Words to know

total cost
What all the items cost together.

change
The money you get back when you pay more than the total.

mental math
Solving in your head, using smart strategies.

count up
Start at the total and jump forward to the money paid. The jumps add up to the change.

compensate
Use a friendly number, then fix the answer. $19 + $8: do $20 + $8 = $28, then take away 1 to get $27.

Remember from Grade 3

In Grade 3 you found the change for one, two or three items by counting up, like a cashier. Now you will do it with more items, bigger prices and payments with $20, $50 and $100 bills, all with mental math.

Let's start with a story

The Community Craft Fair is in the school gym. Local artists are selling their work. The cashier's card machine stops working! The sign now says Cash only. The volunteers need people who can add prices and make change in their heads. Can you help?

Item	Price
Painted mug	$9
Soap bar	$6
Jar of maple syrup	$12
Puzzle	$15
Knitted toque	$16
Wooden birdhouse	$24
Woven basket	$35
Quilt	$85

The Community Craft Fair. Every price is in whole dollars, with no tax.

Strategies for adding prices in your head

Make a ten: look for prices whose ones add to 10. $16 + $24: 6 + 4 = 10, so $16 + $24 = $40.
Split into tens and ones: $24 + $35 → $20 + $30 = $50 and $4 + $5 = $9. Total $59.
Friendly number (compensate): $16 + $9 → $16 + $10 = $26, then take away 1: $25.
Change the order: you can add prices in any order. Add the easy pairs first.

Worked example

Example 1: Make a ten first

Ms. Diaz buys a knitted toque ($16), a wooden birdhouse ($24) and a painted mug ($9). She pays with a $50 bill. What is the total? What is her change?

Look for a ten: 6 + 4 = 10, so $16 + $24 = $40.
Add the mug: $40 + $9 = $49.
Change: count up from $49 to $50. That is $1.
Check: $49 + $1 = $50. ✔

Total $49. Change $1 (a loonie).

Worked example

Example 2: Change from $100

Mr. Chen buys a woven basket ($35), a jar of maple syrup ($12) and a soap bar ($6). He pays with a $100 bill.

Total: $35 + $12 = $47. Then $47 + $6 = $53.
Count up to the next ten: $53 → $60 is $7.
Count up to $100: $60 → $100 is $40.
Add the jumps: $7 + $40 = $47.
Check: $53 + $47 = $100. ✔

Total $53. Change $47.

Count up from $53: jump $7 to $60, then $40 to $100. The change is $7 + $40 = $47.

Give change with the fewest bills and coins

A good cashier uses the biggest bills and coins that fit. Our Canadian bills are $5, $10, $20, $50 and $100. Our dollar coins are the loonie ($1) and the toonie ($2).

For $47: two $20 bills make $40. A $5 bill makes $45. A toonie makes $47. That is only 4 pieces of money.

Mr. Chen's change: two $20 bills, one $5 bill and one toonie make $47.

Worked example

Example 3: Change the order

Jaya buys a puzzle ($15), a painted mug ($9) and a soap bar ($6). She pays with a $20 bill and a $10 bill.

Add the easy pair first: $9 + $6 = $15.
Then $15 + $15 = $30.
She paid $20 + $10 = $30.

Total $30. She paid the exact amount, so there is no change.

Worked example

Example 4: A big purchase

Grandpa buys a quilt ($85) and a wooden birdhouse ($24). He pays with a $100 bill and a $20 bill.

Total: $85 + $15 = $100. There is $24 − $15 = $9 still to add. $100 + $9 = $109.
He paid $100 + $20 = $120.
Count up: $109 → $110 is $1. $110 → $120 is $10. Change: $1 + $10 = $11.
Fewest pieces: a $10 bill and a loonie.
Check: $109 + $11 = $120. ✔

Total $109. Change $11.

Check your change

total + change = money paid. If they don't match, count again. Also ask: does my answer match my estimate?

Stay calm and keep going

Cashiers sometimes feel rushed. Take a breath, say the numbers out loud, and count up step by step. It is fine to jot down a quick note. Careful is better than fast.

Just for fun

Why was the cashier so calm when the card machine stopped working?

She had plenty of change, and she was great at counting up!

Hands-on activity

Craft fair store at home

You need: play money (or paper slips labelled $1, $2, $5, $10, $20, $50 and $100), sticky notes, and 8 things from around the house to "sell".

With a grown-up, write a whole-dollar price on a sticky note for each item. Use prices from $3 to $60.
The shopper picks two or three items and pays with one or two bills.
The cashier adds the prices out loud in their head, then counts up to give change with the fewest bills and coins.
The shopper checks: total + change = money paid.
Switch jobs after each sale. Do at least 6 sales.

Math talk: "Which strategy did you use to add? Was there a ten you could make?"

Grown-up note: coins are a choking hazard. Keep them away from babies and toddlers, and put them away after playing.

Your turn

$16 + $24 = ?

$12 + $9 = ?

Find the total: $35 + $15 + $6.

A puzzle costs $13. You pay with a $20 bill. What is the change?

A total is $38. You pay with a $50 bill. What is the change?

You buy maple syrup ($12) and a knitted toque ($16) with a $50 bill. Find the total and the change.

A total is $64. You pay with a $100 bill. What is the change?

More practice: warm-up

$20 + $15 = ?

$9 + $6 = ?

Change from $10 for a $7 item?

Change from $20 for a $15 item?

$30 + $40 + $5 = ?

More practice: on your own

Buy a birdhouse ($24), a basket ($35) and a mug ($9). Pay with $100. Find the total and the change.

Buy two puzzles ($15 each) and a soap bar ($6). Pay with two $20 bills. What is the change?

$49 + $27 = ? Use a friendly number.

The change is $42. What are the fewest bills and coins the cashier can give?

Ali buys a toque ($16) and a soap bar ($6). He pays $50 and gets $18 back. Is that right?

Challenge

Mrs. Lee buys a quilt ($85), maple syrup ($12) and a mug ($9). She pays with a $100 bill and a $20 bill. What is her change?

Jun buys a basket ($35) and one other item from the craft fair. He pays with a $50 bill and gets $9 change. Which other item did he buy?

Rosa's total is $27. She has a $20 bill, a $10 bill and a toonie. If she pays $30, she gets $3 back. Why might she pay $32 instead?

Check your understanding

Quiz

What is $25 + $25 + $9?

What is $39 + $14?

An item costs $12. You pay with a $20 bill. What is the change?

A total is $73. You pay with a $100 bill. What is the change?

You buy items for $18, $7 and $12. You pay with a $50 bill. What is the change?

Which way gives $15 in change with the fewest pieces of money?

For parents and teachers

What your child is learning: the Ontario curriculum (F1.2) asks Grade 4 students to calculate the cost of transactions with several whole-dollar items (no sales tax) and the change for cash payments, using mental math. This lesson teaches adding prices by making tens, splitting into tens and ones, and using friendly numbers, then counting up to find the change from $20, $50 and $100. Choosing a strategy, checking and persevering are part of the mathematical processes (A1).

Common difficulties:

Leaving out an item when adding three or more prices. Have your child touch or tick each price as it is added.
Fixing a friendly number the wrong way ($49 + $27: adding 1 instead of taking 1 away). Ask, "Did you add too much or too little?"
Losing a jump while counting up. Encourage saying each jump out loud, then adding the jumps.
Giving lots of small coins. Practise using the biggest bills and coins that fit.

How to help: Whenever you pay cash, ask your child to work out the change before the cashier does. Round store prices to whole dollars to keep it at the Grade 4 level (cents and sales tax come in Grade 5). Paper notes are fine; mental math does not have to be silent.