SOLUTION: Hi, I did the following equation and came up with 6. An online program tells me it's 21 without showing me how it got the answer. Plugging in both answers tells me that 21 is r

Question 784897: Hi,
I did the following equation and came up with 6. An online program tells me it's 21 without showing me how it got the answer. Plugging in both answers tells me that 21 is right but I don't know how to work it out step by step. Would you please help me?
3/4 = 12/x-5 I'm not sure it makes a difference but on my homework the numbers are arranged as fractions with the x-5 as the 2nd denominator.
I divided both the left denominator and the right numerator by 4 and got:
3/1 = 3/x-5 or 3=3/x-5
Then I multiplied both sides by x-5 and got:
3x-15 = 3 then I added 15 to both sides and got:
3x = 18 then I divided both sides by 3 and resulted in:
x =6 which I know is wrong but I don't know where I went wrong.
Thanks :)
Answer by Alan3354(69443) (Show Source): You can put this solution on YOUR website!
Expressions-with-variables/784897 (2013-09-12 19:33:16): Hi,
I did the following equation and came up with 6. An online program tells me it's 21 without showing me how it got the answer. Plugging in both answers tells me that 21 is right but I don't know how to work it out step by step. Would you please help me?
3/4 = 12/x-5 I'm not sure it makes a difference but on my homework the numbers are arranged as fractions with the x-5 as the 2nd denominator.
I divided both the left denominator and the right numerator by 4 and got:
3/1 = 3/x-5 or 3=3/x-5
Then I multiplied both sides by x-5 and got:
3x-15 = 3 then I added 15 to both sides and got:
3x = 18 then I divided both sides by 3 and resulted in:
x =6 which I know is wrong but I don't know where I went wrong.
--------------
3/4 = 12/(x-5)
Multiply by 4
3 = 48/(x-5) ****** You divided the numerator on the right side.
3(x-5) = 48
3x - 15 = 48
3x = 63
x = 21
=================
3/4 = 12/(x-5)
Use a shortcut, cross multiply.
3(x-5) = 4*12
etc

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