MathRevolution wrote:
[GMAT math practice question]
(number properties) \(a, b\), and \(c\) are positive integers. Is \(a+b+c\) an odd number?
1) \(ab\) is an odd number
2) \(c\) is an odd number
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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 \(4\) variables (\(a, b, x\) and \(y\)) and \(0\) equations, E is most likely to be the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables with the number of equations, we can save time by considering conditions 1) & 2) together first.
Conditions 1) & 2)
Condition 1), \(ab\) is an odd integer, is equivalent to the statement that both \(a\) and \(b\) are odd numbers.
As \(c\) is also an odd number, \(a + b + c\) is an odd number since it is the sum of three odd numbers.
Both conditions together are sufficient.
Since this question is an integer question (one of the key question areas), CMT (Common Mistake Type) 4(A) of the VA (Variable Approach) method tells us that we should also check answers A and B.
Condition 1)
If \(a = 1, b = 1\), and \(c = 1\), then \(a + b + c = 3\) is an odd number, and the answer is ‘yes’.
If \(a = 1, b = 1\), and \(c = 2\), then \(a + b + c = 4\) is not an odd number, and the answer is ‘no’.
Thus, condition 1) is not sufficient.
Condition 2)
If \(a = 1, b = 1\), and \(c = 1\), then \(a + b + c = 3\) is an odd number, and the answer is ‘yes’.
If \(a = 2, b = 1\), and \(c = 1\), then we have \(a + b + c = 4\) is not an odd number, and the answer is ‘no’.
Thus, condition 2) is not sufficient.
Therefore, C is the answer.
Answer: C
In cases where 3 or more additional equations are required, such as for original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B or D. Since E (i.e. conditions 1) & 2) are NOT sufficient, when taken together) is most likely to be the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2), when taken together. Obviously, there may be occasions on which the answer is A, B, C or D.
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