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Lesson · Grade 8 · Mathematics

Best buy: comparing unit prices

  • Grade 8
  • Mathematics
  • 2 min 22 s
  • British Columbia
  • BC8-MA-43

Two boxes of the same cereal, 750 g for $4.29 and 1.2 kg for $6.49, stay on screen while both amounts are put in grams and each price is divided by its number of 100 g groups, giving $0.572 against about $0.541 per 100 g, with each result checked by multiplying back. The big box is the better buy as long as it is eaten before it goes stale, and the video ends on a three-step recap: use the same units, work out each unit price, and choose the lowest one you will use up.

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In one sentence

The question this lesson answers

Two boxes of the same cereal: the small box holds 750 grams for 4.29 dollars, the big box holds 1.2 kilograms for 6.49 dollars.

Which is the better buy, the one that gives more cereal for your money?

Transcript

What the lesson says

Two boxes of the same cereal: the small box holds 750 grams for 4.29 dollars, the big box holds 1.2 kilograms for 6.49 dollars.

Which is the better buy, the one that gives more cereal for your money?

The small box costs less, but it holds less. Comparing prices without the amounts is a common mistake.

Instead, compare unit prices: the cost of one fixed amount, here 100 grams.

First, use the same unit, so we do not mix grams and kilograms. The small box is already in grams.

One kilogram is 1000 grams, so 1.2 kilograms is 1.2 times 1000, which is 1200 grams.

Next, count the groups of 100 grams: 750 divided by 100 is 7.5, and 1200 divided by 100 is 12.

Then divide each price by its number of groups. For the small box, 4.29 divided by 7.5 is 0.572 dollars per 100 grams.

Check: 0.572 times 7.5 is 4.29, the price on the box.

For the big box, 6.49 divided by 12 is 0.54083 and so on, which is approximately 0.541 dollars per 100 grams.

Check: 0.541 times 12 is 6.492, which rounds to 6.49, the price on the box.

0.541 dollars is less than 0.572 dollars, so 100 grams costs less in the big box.

So the big box is the better buy, if it is eaten before it goes stale.

To find the best buy, use the same units, work out each unit price, and choose the lowest one you will use up.

Curriculum

Where this fits

British Columbia, Grade 8

Mathematics, Financial literacy: financial literacy: best buysBC8-MA-43

Next: Coupons and deals: which offer saves more?

The outcomes and strands follow the British Columbia curriculum.

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