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Lesson 6.3 Writing Proportions

Lesson 6.3 Writing Proportions. Students will be able to use proportions to solve problems. Use a proportion to solve the problem. You are reading a 220 page book. It takes you 15 minutes to read the first 10 pages. How long will it take you to read the entire book?.

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Lesson 6.3 Writing Proportions

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  1. Lesson 6.3 Writing Proportions Students will be able to use proportions to solve problems

  2. Use a proportion to solve the problem. You are reading a 220 page book. It takes you 15 minutes to read the first 10 pages. How long will it take you to read the entire book? Remember a proportion is a comparison of two ratios, and a ratio is a comparison of two quantities. What two quantities are being compared? x Minutes pages OR! 220 Pages minutes 15 minutes 10 pages 10 pages 15minutes = = x 220 15 x 220 = 10x 15 x 220 = 10x This proportion may have also been written: 15 minutes = 10 pages X minutes 220 pages The second ratio in the proportion must use the same “comparison”! In the second ratio one value will be given and one will be the unknown OR x. Use cross products to solve.

  3. In the previous slide you were shown threeways to set-up a proportion. In the following examples tell which of the proportions you COULD NOTuse to solve the problem. You are designing a new postage stamp. The stamp must be 3 cm tall by 2 cm wide. You make a sketch of the stamp that is 6 inches tall. How wide should the sketch be? wide 3cm 6in 2cm xin tall tall B) 3cm 2cm 6in xin tall wide C) 3cm 2cm xin6in Tall = = = wide wide tall wide wide tall Hint: Make sure the comparison of quantities are the same in BOTH the ratios used in the proportion. ( And true!) One gallon of paint covers 900 square feet. What area does one quart of the same paint cover? A) 4 qt 1 qt 900 ft2 x ft2 B) 1 qt 900 ft2 4 qt x ft2 = C) 4qt 900ft2 1qt xft2 = = Remember 1 gal = 4qt

  4. Solve the Proportion If 24 pounds of fertilizer will cover 1750 square feet, how many pounds of fertilizer will be needed to cover 6562.5 square feet? First determine one comparison. OR 1750 ft2 24 lbs 24 lbs 1750 ft2 OR 24 lbs X lbs OR 24 lbs = 1750 ft2 Set-up the second ratio, using the SAME comparison of units! 24 lbs 1750 ft2 X 6562.5 lbs ft2 = Use cross products to solve. X= 90 lbs 24 x 6562.5 = 1750x : : 157500 = 1750x 1750 1750

  5. When solving problems with proportions the proportion may be written correctly several different ways. The key to correctly writing proportions is that the comparison of units be the same in BOTH ratios within the proportion. (That is the reason why you must always write in the units!)

  6. Homework Time Handout E10 Must do work on another piece of paper No work No Credit!

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