Asad
x is a positive integer. Is it odd or even?
(1) \(x^2+x\) is even
(2) \((x + 2) (x – 4)\) is even
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.
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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.
Since \(x^2+x = x(x+1)\) is a product of two consecutive integers, it is always an even number whatever the integer \(x\) is.
Thus, condition 1) doesn't tell anything about the parity of \(x\).
Since condition 1) does not yield a unique solution, it is not sufficient.
Condition 2)
\((x+2)(x-4)\) is even
⇔ \(x+2\) is even or \(x-4\) is even.
⇔ \(x\) is even since both "\(x+2\) is even" and "\(x-4\) is even" are equivalent to "\(x\) is even" since they are the expressions of adding or subtracting even numbers \(2\) and \(4\) from \(x\).
Since condition 2) yields a unique solution, it is sufficient.
Therefore, B is the answer.