[GMAT math practice question]
What is the range of the 6 numbers x, y, 8, 10, 14, and 16?
1) Their average (arithmetic mean) is 12
2) 8<x<y<16
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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.
Since we have 2 variables (x and y) and 0 equations, C is most likely to be the answer, and so we should consider conditions 1) & 2) together first.
Conditions 1) & 2):
Condition 1) states that ( x + y ) / 2 = 12 or x +y = 24.
Condition 2) tells us that 8 < x < y < 16. So, x can be no smaller than 8, and y can be no larger than 16. Therefore, the range of x,y,8,10,14,16 is 8 = 16 – 8.
Since this is an inequality question (one of the key question areas), we should also consider choices A and B by CMT 4(A).
Condition 1)
If x = 7 and y = 17, the range is 17 – 7 = 10.
If x = 9 and y = 15, the range is 16 – 8 = 8.
As we do not have a unique answer, condition 1) is not sufficient.
Condition 2)
Since 8 < x < y < 16, the maximum value in the list is 16, and the minimum value in the list is 8.
Therefore, the range is 8 = 16 – 8.
The answer is A.
Normally, in problems which require 2 or more additional equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
Answer: B
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