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# I need help on proportions

I learned about this in class today but i am not so good at math so of course i didnt under stand

3/7=a/21

### 3 Answers by Expert Tutors

Timothy B. | Math (Algebra, Calculus), Physics, Chemistry - 6th-12th gradeMath (Algebra, Calculus), Physics, Chemi...
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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.

Rick F. | "Math and Spanish tutor. Fifteen years exp./MEd"Math and Spanish tutor. Fifteen years ...
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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!