Lesson 1.16, one day, 13 slides. All prices marked [update from this week's circular].
Do now
- A 16 oz box of pasta costs $1.29.
- A 12 oz box of a different brand costs $1.49.
- Which is the better buy? Show your thinking.
Teacher notes
Most pick $1.29 because the number is smaller. Ask: smaller because it is a better deal, or because there is less in the box? Here it is both. The next one is harder.
Now this one
- 24 oz jar of sauce: $2.29
- 45 oz jar of the same sauce: $3.79
- The big one costs more. Is it the better buy?
- You cannot tell by looking.
Teacher notes
Image: the two jars side by side with the prices.
The rule
- Unit price = price divided by quantity
- $2.29 divided by 24 oz = $0.095 per oz
- $3.79 divided by 45 oz = $0.084 per oz
- The 45 oz jar wins per ounce.
- Write the dollar sign on every answer. No dollar sign means you divided the wrong way.
The store already did the math
- The shelf tag prints the unit price in small letters.
- The sale price is in big letters.
- Guess which one the store wants you to read.
Teacher notes
Image: a photo of a real supermarket shelf tag with the unit price circled.
Units have to match
- 1 pound = 16 ounces
- $4.99 for a 2 lb bag = $4.99 for 32 oz = $0.156 per oz
- Never compare a per-pound price to a per-ounce price. Convert first.
Store brand vs name brand
- Often made in the same factory with a different label.
- Usually 20 to 40 percent less.
- Thumb up if you think the store brand wins per ounce.
Teacher notes
Image: a store-brand and a name-brand box of the same pasta.
Reveal with the real packages and the math.
Small vs bulk
- Bulk usually wins per ounce.
- Bulk only wins if you can use it before it spoils and have a place to keep it.
- Thumb up for the big bag.
Teacher notes
Image: an 8 oz bag and a 2 lb bag of shredded cheese.
Sales tax at the register
- In New York, most grocery food is not taxed.
- Foil, paper plates, cups, soap: taxed.
- Candy, soda, prepared hot food: taxed.
- Our county: about 8.625 percent. [Sal: confirm the local rate]
- A $3.49 roll of foil rings up at $3.79.
Practice
- Part A: six unit prices. Part B: three better buys. Part D: the bulk trap.
- Pairs. Calculators. 12 minutes.
- Convert pounds to ounces before you compare.
- Peer checkers: help, do not tell.
The bulk trap
- 10 lb of potatoes for $5.99. 3 lb for $2.99.
- This family eats potatoes twice a month, 2 lb each time, and keeps them in a warm cabinet.
- Which bag?
Teacher notes
Take answers. Build the list on the next slide from what they say.
When cheaper is not better
- It will spoil before you use it.
- There is no room to store it.
- The family will not eat that much.
- A food rule or allergy rules it out.
- You cannot afford the bigger total today, even though it is cheaper per ounce.
Teacher notes
The last one is the hardest and the most real. Say it plainly.
Exit card
- 3 steps to find a unit price
- 2 reasons the cheaper unit price can be the wrong buy
- 1 item your family buys where you now want to check the shelf tag
Tomorrow
- You bought it. You got it home. Where does it go?
- Bring back the answer to: what temperature should a refrigerator be?
Teacher notes
Image: an open refrigerator, shelves empty.