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 jar contains red and green marbles, available in two sizes- small and big. 20% of the marbles are red and 60% of the marbles are small. How many small green marbles does the jar contain?

(1) The total number of green marbles is 80.

(2) 10% of total marbles in the jar are small and red

Transforming the original condition and the question, we have the below 2by2 table that is common in GMAT math test.

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GC DS alphonsa A jar contains red and green marbles(20150918).png [ 2.78 KiB | Viewed 682 times ]
there are 4 variables (a,b,c,d) and 2 equations (a+c=20%(a+b+c+d), a+b=60%(a+b+c+d)) therefore we need 2 more equations to match the number of variables and equations. Since there is 1 each in 1) and 2), C has high probability of being the answer and it turns out that C actually is the answer.

Normally for cases where we need 2 more equations, such as original conditions with 2 variable, or 3 variables and 1 equation, or 4 variables and 2 equations, we have 1 equation each in both 1) and 2). Therefore C has a high chance of being the answer, which is why we attempt to solve the question using 1) and 2) together. Here, there is 70% chance that C is the answer, while E has 25% chance. These two are the key questions. 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 according to DS definition, we solve the question assuming C would be our answer hence using ) and 2) together. (It saves us time). Obviously there may be cases where the answer is A, B, D or E.

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