Investigate and calculate ‘best buys’, with and without digital technologies (ACMNA174)

LO: To calculate which item is a better buy.


  • What percentage is.

  • How to calculate the percentage of an item.

  • How to calculate the unit price of an item.

  • How different units of measurement is used to measure different quantities. eg.) ml for capacity/volume, g for mass.


  • That percentage can be used to represent a number.

  • That the bigger the percentage discount doesn’t necessarily mean it’s the better-discounted price.


  • I can calculate which item is a better buy.

Essential Questions:

    • What is a percentage?

    • Why is a percentage always out of 100?


  • 100% of an item means the full number.

  • A percentage of a number means, finding a part of the number.

Finding the Percentage of an Amount

Remember a percentage is just a fraction out of 100.

For example, 20% can be written as 20 out of 100.

When I multiply 20/100 by 45/1, I can simplify it to 45/5 = 9

So 20% of 45 is 9.

Calculating Best Buys

To calculate best buy is fairly simple. All you have to do is calculate the final price of the 2 items then figure out which is the cheaper option. Remember the higher the discount doesn’t mean that you actually walk out of the store paying less!

Calculating Unit Price

To calculate unit price, you have to break down the item to the lowest single unit.

In this case, you have to pay $4.57 for 2 litres of orange juice, so $4.57 / 2 L = $2.29 per litre of orange juice.

Calculating Best Buy Using Unit Price

Option 1
$12.99 for 5 kg of rice. $12.99 / 5 = $2.60 per kg of rice.

Option 2
$3.99 for 1 kg of rice. $3.99 per kg of rice.

Therefore Option 1 is the “better” option.

This is called the unitary method for finding ‘best buys’. Basically, this strategy is to find the price of the basic unit, then using that information to find out further what is needed.


Best Buy Videos

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