Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

Given that x and y are positive integers, what is the units digit of (5*x*y)^(289)?

(1) x is odd.

(2) y is even.

When you modify the original condition and the question, if x or y is even from 5xy, once place of 5xy is always 0. Then, you need to figure out if x or y is odd. In 1), x is odd, which is unique and sufficient.

Thus, the answer is A.

This is one of the mistake type questions. Getting this kind of question right leads you to the score range of 50-51.

Once we modify the original condition and the question according to the variable approach method 1, we can solve approximately 30% of DS questions.

For cases where we need 2 more equations, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.

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