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← Writing Proportions

Setting up proportions to solve a word problem

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Showing Revision 4 created 02/21/2013 by Andras Fuchs.

  1. I have three word problems in this video.
  2. What I want to do in this video is not solve the word problems,
  3. but setup the equation that we could solve to get the answer to the word problem.
  4. What we are essentially going to do is setup the proportionality for the word problems.
  5. So In this 1st problem
  6. we have 9 markers cost $11.50. And then they ask:
  7. How much will 7 markers cost.
  8. Now, let's just set X to be equal to our answer, where X is equal to the cost of 7 markers.
  9. The way to solve a problem like this is to setup two
  10. ratios and set them equal to each other.
  11. So you could say that the ratio of 9 markers
  12. to the cost of 9 markers; 9/ 11.50 = 7/ X
  13. This is a completely valid proportion here.
  14. You could solve this to figure out how much those
  15. 7 markers will cost.
  16. You could have 11.50/9 = X/7 .This is also a valid ratio.
  17. You could also think about ratios in other ways
  18. You could say, that the ratio of 9 markers to 7 markers,
  19. is going to be same as the ratio of their cost
  20. 9/7 = 11.5/X or 7/9 = X/11.5
  21. So all of these would be valid proportion.
  22. Sp let's do this problem now. 7 aples cost $5.
  23. How much can I buy with $8.
  24. How many apples - let's call that X. We need to solve for X
  25. So we have the ratio between number
  26. of apples and cost of the apples - 7/5= X/8
  27. In this first situation the unknown was cost, in this example
  28. the unknown is number of apples.We can do all the different
  29. scenarios as above. we could say 7/X = 5/8
  30. Now lets do the last one. We have a cake recipe for
  31. 5 people requires 2 eggs. How many eggs? So we want to know how many eggs?
  32. We will call how many eggs, which we
  33. need to find out as X, we can call it anything Y,A,B,C anything.
  34. So you could say the ratio of people to eggs is constant.
  35. We have 5 people for 2 eggs - 5/2 = 15/X
  36. Or you could say the ratio between 5/15 = 2/X
  37. All of these we setup the proportion and we can
  38. solve for X and get the answer.