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.

If a and b are both two digit integers, is a + b a multiple of 11?

(1) The tens digit of a is equal to the units digit of b, and the tens digit of b is equal to the units digit of a

(2) Both a and b are odd

In the original condition, there are 2 variables(a,b), which should match with the number of equations. So you need 2 equations. For 1) 1 equation, for 2) 1 equation, which is likely to make C the answer. When 1) & 2), they become a=10x+y=odd and b=10y+x=odd -> a+b=10x+y+10y+x=11(x+y), in which 11 is the factor. Thus, it is sufficient and C is the answer.

However, this is an integer question, which is one of the key questions. Apply the mistake type 4(A). Even just for 1), 11 is always the factor, which is yes and sufficient.

Thus, the answer is A.

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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