The last part of the example can’t be worked out in terms of $/ egg. Using the information given, you can only calculate the cost per serving. A box of ready-made egg white omelette with low fat cheese that sells for $3.99 and contains two servings: $3.99 ÷ 2 = $1.995 = $1.995/serving (this answer is not rounded) At 20.5¢/egg, the best buy is to purchase 18 extra-large white eggs for $3.69. Choosing Units for Unit Rates Which is the better buy? A 750 mL can of tomato soup for $5.99, or a 284 mL can of tomato soup for $2.39? The price per can of soup will not help you decide which is the better buy, because each can contains a different amount of soup. So how can you choose? Instead of calculating the price per can, you should calculate the price per mL. Calculate the price per mL of soup. Example 4 $5.99 750 mL $2.39 284 mL
Mathematics for Everyday Life MEL3E-B Lesson 8, page 7 Copyright © 2008 The Ontario Educational Communications Authority. All rights reserved. Solution The large can of tomato soup: $5.99 ÷ 750 mL = $0.00799 = $0.00799/mL or 0.80¢/mL The small can of tomato soup: $2.39 ÷ 284 mL = $0.0084154 = $0.0084154/mL or 0.84¢/mL The large can of tomato soup has a lower unit rate, so it is the better buy. Sometimes it is hard to compare items that sell for less than a penny per unit, such as the example above. It is often better to calculate the cost per 100 mL or 100 g, or other multiples of the given unit. You could have calculated the example above for the price per 100 mL and it would have looked like this: The large can of tomato soup: $5.99 ÷ 750 mL = $0.00799 = $0.0080/mL × 100 mL = $0.80/100 mL or 80¢/100 mL The small can of tomato soup: $2.39 ÷ 284 mL = $0.0084154 = $0.0084/mL × 100 mL = $0.84 /100 mL or 84¢/100 mL It’s easy to see that at 80¢/100 mL the large can of tomato soup is cheaper than the small can of soup. Note: In big supermarkets, the unit price is often shown next to the item price. There’s More to Life Than Money Calculating the unit rates in the egg example told you that the best buy was 18 extra-large white eggs for $3.69. It is the cheapest price listed for one egg. But is it really? What if you live alone and only eat one egg for breakfast on the weekends? Does it make sense for you to buy 18 eggs? Using
Lesson 8, page 8 Mathematics for Everyday Life MEL3E-B Copyright © 2008 The Ontario Educational Communications Authority. All rights reserved. two eggs a week means that it will take you about nine weeks to use all the eggs. Do you want to eat a nine-week-old egg? Do you even want to have such old eggs in your refrigerator? This is where common sense tells you to buy the package of six eggs, even if it is slightly more expensive. The decision then comes down to whether you will spend more money on eggs at the higher price, but smaller package, than you will by having to replace the old eggs you can’t use. Look at the previous example, assuming that three-week-old eggs are still edible.
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