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.
A certain expressway has Exits J, K, L, and M, in that order. What is the road distance from Exit K to Exit L ?
(1) The road distance from Exit J to Exit L is 21 kilometers.
(2) The road distance from Exit K to Exit M is 26 kilometers.
In the original condition, if we let the distance between J&K=a, K&L=b, L&M=c, the question is asking about the value of b, so there are 3 variables and 2 equations are given from the conditions, so there is high chance (E) will be our answer.
Looking at them together, a+b=21, b+c=26, we cannot get a unique value for b, so this is insufficient, and the answer is (E).
For cases where we need 3 more equation, such as original conditions with "3 variables", or "4 variables and 1 equation", or "5 variables and 2 equations", we have 1 equation each in both 1) and 2). Therefore, there is 80% chance that E is the answer (especially about 90% of 2 by 2 questions where there are more than 3 variables), while C has 15% 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 E 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, C or D.
