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What is the average of

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What is the average of

by Max@Math Revolution » Tue Mar 10, 2020 12:17 am

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[GMAT math practice question]

What is the average of \(x_1,x_2,...,x_{10}\) ?

1) The average of \(x_1,x_2,...,x_8\) is 5.
2) \(x_9=9-x_{10}\)
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Source: — Data Sufficiency |
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Re: What is the average of

by Max@Math Revolution » Thu Mar 12, 2020 12:12 am

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=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

Condition 1) tells us that ( x1 + x2 + … + x8 ) / 8 = 5 or x1 + x2 + … + x8 = 40.
Condition 2) tells us that x9 + x10 = 9.
Thus, when we consider both conditions together, we have ( x1 + x2 + … + x8 + x9 + x10 ) / 10 = (40 + 9) / 10 = 49/10 = 4.9.

Therefore, C is the answer.
Answer: C
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