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.

To receive a driver license, sixteen year-olds at Culliver High School have to pass both a written and a practical driving test. Everyone has to take the tests, and no one failed both tests. If 30% of the 16 year-olds who passed the written test did not pass the practical, how many sixteen year-olds at Culliver High School received their driver license?

(1) There are 188 sixteen year-olds at Culliver High School.

(2) 20% of the sixteen year-olds who passed the practical test failed the written test.

This is a "2by2" question, one of the most common type of question in GMAT math

we get a table as below:

Attachment:

GCDS iamba To receive a driver license (20151126).jpg [ 44.75 KiB | Viewed 2509 times ]
There are 3 variables (a,b,c) and one equation (c=0.3(a+c)) in the original condition, and 2 equations are given by the conditions, so there is high chance (C) will be the answer

Looking at the conditions together,

a+b+c=188, b=0.2(a+b), and this is sufficient to achieve an answer,

so the answer becomes (C).

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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