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GMAT Algebra http://www.800score.com/forum/viewtopic.php?f=3&t=15349 
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Author:  questioner [ Mon Apr 29, 2013 11:34 am ] 
Post subject:  GMAT Algebra 
If 1/3 + 1/2 + 1/x = 4, then x = A. 5/18 B. 6/19 C. 24/11 D. 19/6 E. 18/5 (B) To solve this equation, we first need to find a common denominator to simplify the equation. The lowest common denominator is 3 × 2 × x = 6x. If we multiply the both sides of the equation by 6x, we can get rid of all of the fractions. Multiplying 1/3 + 1/2 + 1/x = 4 by 6x, we get: 6x/3 + 6x/2 + 6x/x = 4(6x). Reducing this, we get: 2x + 3x + 6 = 24x 5x + 6 = 24x 6 = 19x, or x = 6/19. Now you might be tempted to stop there, but if you have extra time you can double check by backsolving in for x. You can plug in 6/19 for x. 1/(6/19) is the same as 19/6. 1/2 = 3/6 and 1/3 = 2/6 so you can put everything in terms of 6ths and get: 2/6 + 3/6 + 19/6 = 24/6 = 4. So, you can see with 100% certainty that the answer of 6/19 is correct. The correct answer is choice (B).  Hi. I didn't understand why we have to multiply both sides of the equation. What I did is finding LCD that is 6, and I did not multiply 4 by 6... Also, I didn't understand what is 19/6 ? Thanks! 
Author:  Gennadiy [ Mon Apr 29, 2013 11:35 am ] 
Post subject:  Re: GMAT Algebra 
Quote: I didn't understand why we have to multiply both sides of the equation. We do that to get rid of the denominators.Quote: What I did is finding LCD that is 6, and I did not multiply 4 by 6... 6 is the LCD of the first two fractions only. Knowing it you can add 1/3 and 1/2 .1/2 + 1/3 = 3/6 + 2/6 = (3 + 2)/6 = 5/6 The original equation becomes: 5/6 + 1/x = 4 There is still a way to go. There will be a multiplication of 4 by 6 somewhere along it. Quote: Also, I didn't understand what is 19/6 ? We got the answer x = 6/19 . If we want to check that we did no mistakes along the way and this answer is in fact the correct one, we can plug x = 6/19 in the original equation.1/3 + 1/2 + 1/(6/19) ?= 4 If this equality is true, then the solution x = 6/19 is correct. The third term, 1/(6/19) , equals 19/6 . This is where we get 19/6. 1/3 + 1/2 + 19/6 ?= 4 The remaining step is to sum the fractions and see if we get 4. 
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