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# Writing proportions example

Some examples of writing two ratios and setting them equal to each other to solve proportion word problems. Created by Sal Khan.

Video transcript

I have three word problems in this video. What I want to do in this video is not solve the word problems, but setup the equation that we could solve to get the answer to the word problem. What we are essentially going to do is setup the proportionality for the word problems. So In this 1st problem we have 9 markers cost $11.50. And then they ask: How much will 7 markers cost. Now, let's just set X to be equal to our answer, where X is equal to the cost of 7 markers. The way to solve a problem like this is to setup two ratios and set them equal to each other. So you could say that the ratio of 9 markers to the cost of 9 markers; 9/ 11.50 = 7/ X This is a completely valid proportion here. You could solve this to figure out how much those 7 markers will cost. You could have 11.50/9 = X/7 .This is also a valid ratio. You could also think about ratios in other ways You could say, that the ratio of 9 markers to 7 markers, is going to be same as the ratio of their cost 9/7 = 11.5/X or 7/9 = X/11.5 So all of these would be valid proportion. Sp let's do this problem now. 7 aples cost $5. How much can I buy with $8. How many apples - let's call that X. We need to solve for X So we have the ratio between number of apples and cost of the apples - 7/5= X/8 In this first situation the unknown was cost, in this example the unknown is number of apples.We can do all the different scenarios as above. we could say 7/X = 5/8 Now lets do the last one. We have a cake recipe for 5 people requires 2 eggs. How many eggs? So we want to know how many eggs? We will call how many eggs, which we need to find out as X, we can call it anything Y,A,B,C anything. So you could say the ratio of people to eggs is constant. We have 5 people for 2 eggs - 5/2 = 15/X Or you could say the ratio between 5/15 = 2/X All of these we setup the proportion and we can solve for X and get the answer.