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I need help on proportions

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3 Answers

You wrote your question perfectly, Giselle. Now let's think about this together.
For example:
If it take 3 trees to make 7 tables...
Then it takes (a) trees to make 21 tables.

How many trees make 21 tables? Well, if we write that in mathematical terms we see 3/7 = a/21 (3trees per 7 tables, etc...)
Moving each denominator to the other side of the equal sign we get --> 3(21) = 7a
Solve for a.

Giselle, It's not so much about being good at math, it's more about practicing over and over. You can do it! I hope this helps. :) 3/7 = a/21 Write the fraction in vertical form(I can't do it on here)with the 3 over the 7 and the a over the 21. Draw a line from the 3 to the 21 by going through the equals sign. Draw a line from the a to the 7 by going through the equals sign. What you should see now is a big 'x' or cross. This is where the term 'cross multiplication' comes from, because now you are going to multiply 3(21) and 7(a). Write an equation with the products of your multiplication equal to each other. 3(21)= 7(a) 63 = 7a Now, divide each side by 7. Write the results of your division. 63 divided by 7 equals 9. 7a divided by 7 equals a. 9 = a is your answer. To check your solution, put the value you found for a(9) in the original proportion and go through the same process. 3/7 = 9/21 Cross multiply 3(21) = 7(9) 63 = 63 is a true statement, so your answer was correct! Good luck!

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Ask yourself, what did I do to 7 to get to 21. Answer: Multiply it by 3. So you need to do the same thing to the numerator. 3x3=9. a=9