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.
Is the integer x a multiple of 10?
(1) when x is divided by 5, the result an even integer.
(2) x + 20 is a multiple of 10.
There is one variable (x) and 2 equations are given by the conditions, so there is high chance (D) will be the answer.
For condition 1, x=5p+even (p:positive integer), and the answer to the question becomes 'yes' for x=10, but 'no' for x=7, so this is insufficient.
For condition 2, from x+20=10q(q:positive integer), x=10q-20=10(q-2)= a multiple of 10. This answers the question 'yes' and is sufficient. Therefore, the answer becomes (B).
For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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