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Use ratio reasoning to convert measurement units; manipulate and transform units appropriately when multiplying or dividing quantities.
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- Small baby bottles (with fluid ounces measured on sides)
- Dried items such as lentils, beans, irish oats, or chickpeas
- Water and pots
- Cooking equipment (optional)
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Note: for this lesson, it helps if students have some experience dividing and multiplying fractions
Step One: Introduction to Adjusting a Recipe with Ratios
Explain to students that they will practice adjusting ingredients to match serving sizes and convert units measured in cups to ounces.
Explain to students that when cooking, it often makes sense to be able to adjust recipes and be flexible with measuring tools. For example, someone making Irish oats for breakfast might want to make a single serving (as opposed to the serving suggestion which is intended for several people), and might only have a baby bottle available for measuring (which lists ounces, but not cups).
If a recipe calls for a cup of oats and four cups of water to serve four people, the person cooking would need to know that for a single serving, she would divide the numbers by four to get a single serving. To do this using a ratio, the cook might write the following:
4 cups water: 1 cup oats
4 cups water ( ÷ 4) = 1 cup water
1 cup oats (÷ 4) = ¼ cup oats
For one serving, the recipe calls for a ratio of 1: ¼ (1 cup of water: ¼ cup oats)
Using their bags of dried foods, students will calculate the ratio of water to food for a single serving size of the food item.
Step Two: Using Ratios to Convert Units of Measurement
Write the following ratio on the board:
1 cup = 8 ounces
Ask students to pair up and take a package of dried items, a baby bottle and water.
Explain to students that it is important to know the number of ounces in a cup so that in a situation where a recipe calls for measurement in cups (but they can only measure in ounces), they can still accurately follow a recipe.
Demonstrate how to convert cup measurements to ounces by creating ratios:
1 cup = 8 oz.
If 1 cup = 8 oz., how many ounces are in ½ cup? ¼ cup?
1: 8 = ½: x (half of 1 is ½; half of 8 is 4, so there are 4 ounces in ½ cup)
1:8= ¼: x (one quarter of 1 is ¼, one quarter of 8 is 2, so there are 2 ounces in ¼ cup)
Note: if teacher and students are comfortable using the cross-multiplication method of solving a proportion, this may be a useful method in converting units as well)
For hands-on practice, students can pour water or dried food into the baby bottles to demonstrate each measurement ratio, measuring using the ounce markers on the baby bottle and noting how many ounces are in each cup-based measurement. If you are comfortable with students marking on baby bottles, it may help them to mark each cup measurement physically on the bottle at the appropriate ounce marker.
Step Three: Ratio and Hands-On Measurement Practice
Give students time to practice writing ratios for their measurements and converting the measurements into ounces before measuring out the dried food items and adding them to pots with water. Teacher will ask students to convert the measurements on the bags to make a single serving, a serving size for two, three and four. Students will divide or multiply the ratio of dried food items to water by the appropriate numbers to match serving sizes, and will then use ratios to convert cup measurements into fluid ounces.
Serving of 4= 1 cup lentils to 3 cups water (given on bag)
Serving of 1: Divide ratio of 1: 3 by 4 to find one serving size, which gives you ¼ cup lentils and ¾ cup water
Convert cups to ounces: 1 cup: 8 oz.
Single serving: 1:8 = ¼:2 (2 ounces of lentils) 1:8 = ¾: 6 (6 ounces of water)
Single serving ratio of lentils to water (in ounces): 2 oz. lentils : 6 oz. water (2:6)
Depending on your preference, students can just practice measuring and pouring dried food and water into pots, or you may want to provide the opportunity to cook and taste the items to see if students measured correctly (rice cookers and vegetable steamers work well in this situation).
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Assess student learning by observing their ability to write ratios to adjust serving sizes and convert units. You may want to administer a short quiz or practice sheet to see if students can apply the concept to solving similar math problems, as well.
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Allow students to bring in their own recipes to convert into different serving sizes and measurements (from cups to ounces, or vice versa). Correlate these math skills with a lesson on healthy eating and cooking at home to help students build better habits.