Begin your lesson plan with a review of ratios. Plan to use manipulatives as a demonstration. Prepare a bag of red and blue marbles, and plan to have students tell you the ratio of red marbles to blue marbles in the bag and the ratio of red marbles to the total number of marbles in the bag.
Write a lesson on proportions, explaining that a proportion is an equation that compares equal ratios. Explain that you know two ratios are a proportion when you can cross-multiply them and get equal numbers. For example, 2/4 = 3/6 is a proportion because 2 x 6 = 12 and 3 x 4 = 12, proving the proportion true.
Write an example using doubling a recipe, explaining that as long as a cook keeps the ratios in proportion, he can make the recipe as big as he wants. For example, choose a recipe that serves two people and calls for 1 cup of flour and 2 cups of sugar. To serve four people, a cook must double both sides of the ratio, making 2 cups flour and 4 cups sugar. Plan to have students cross-multiply to test the proportion.
Explain in your lesson that students can find missing numbers in ratios by cross-multiplying and then solving for the variable. Using the recipe in Step 3, plan to ask the students to find how many cups of flour the cook needs if he wants to use 8 cups of sugar. Have them cross-multiply 1/2 = x/8 to get 8 = 2x, and then have them solve for x to get x = 4.
Write a set of practice word-problems using cooking examples and unit conversation examples. For example, have students solve for the number of inches in 50 feet if there are 12 inches in 1 foot.