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GMAT Algebra (Data Sufficiency) http://www.800score.com/forum/viewtopic.php?f=3&t=15334 
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Author:  questioner [ Wed Apr 17, 2013 9:27 am ] 
Post subject:  GMAT Algebra (Data Sufficiency) 
What is the average (arithmetic mean) of 3x and 15z? (1) x + 5z = 20 (2) x + z = 8 My question: I do not understand how you can figure our the first value but not the second... Isnt it set up the same 
Author:  Gennadiy [ Wed Apr 17, 2013 9:27 am ] 
Post subject:  Re: GMAT Algebra (Data Sufficiency) 
If we multiply everything in (1) by 3, we will get the sum for 3x + 15z = 60. So then we can simply divide 60 /2 = 30 to get the avg. If we multiply (2) by 3 we get 3x + 3z = 24, we are not interested in 3x and 3z, we are interested in 3x and 15z. Similarly, if we multiply (2) by 15 we get 15x + 15z = 120. Again, that tells us nothing about the sum of 3x and 15z. Take care. 
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