Proportions How to Write and Solve Them. PROBLEM: I saw at Office Depot that pencils were priced at a dozen for $1.50 How much is it for one pencil? How.

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Presentation transcript:

Proportions How to Write and Solve Them

PROBLEM: I saw at Office Depot that pencils were priced at a dozen for $1.50 How much is it for one pencil? How much is it for 8 pencils? How much is it for 20 pencils? How many pencils can I buy for $2 ? How many pencils can I buy for $5 ?

Problems like these can be solved using proportions... What are proportions? How do you solve them?

Proportion: an equation showing two equal ratios... Examples: = = x3x = x4x = =

Writing (Setting Up) Proportions Write a rate (ratio) from the information you know: 12 pencils $1.50 Choose a variable for the unknown Write a second ratio, keeping the units in the same order.

12 pencils $1.50 Choose a variable for the unknown Write a second ratio, keeping the units in the same order. How much is it for one pencil? = 1 pencil X Write the rate

How to solve proportions... METHOD ONE: Look for a pattern and use “guess and check” METHOD TWO: Cross multiply to form a new equation Solve this equation by dividing Check your answer

METHOD ONE Example: 12 pencils $1.50 = 1 pencil X Divided by 12 Looking for a Pattern..

METHOD TWO Example: 12 pencils $1.50 = 1 pencil X Cross Multiply … 12x =$ 1.50 Solve for x 12

How much is it for 8 pencils? How much is it for 20 pencils? How many pencils can I buy for $2 ? How many pencils can I buy for $5 ? Try to work these out in your notes...