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.

In a group of 1000 people, each person is either left-handed or right-handed but not both. 80% of the females are left-handed, and 84% of the right-handed people are male. How many people are right-handed?

(1) There are 600 males in the group

(2) There are 180 left-handed males in the group

From the original condition and the question, we can obtain the below 2by2 table that is common in GMAT math test

Attachment:

GC DS Bunuel In a group of 1000 people each(20150917).jpg [ 23.06 KiB | Viewed 1940 times ]
In the above table we have 4 variables (a,b,c,d), 3 equations(a+b+c+d=1000, a=0.8(a+c), b=0.94(a+b)) and thus we need 1 more equation to match the number of variables and equations. Since there is 1 each in 1) and 2), D has high probability of being the answer and it turns out that D actually is the answer.

Normally 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 D has a high chance of being the answer, which is why we attempt to solve the question using 1) and 2) separately. Here, there is 59 % chance that D is the answer, while A or B has 38% chance. There is 3% chance that C or E is the answer for the case. Since D is most likely to be the answer according to DS definition, we solve the question assuming D would be our answer hence using 1) and 2) separately. Obviously there may be cases where the answer is A, B, C or E.

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