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
Of the 60 families in a certain neighborhood, 38 have a cat. How many of the families in this neighborhood have a dog?
(1) 28 of the families in this neighborhood have a cat but not a dog
(2) The number of families in the neighborhood who have a dog and a cat is the same as the number of families who have neither a cat nor a dog.
This is a “2-by-2” question, one of the most common types of questions in GMAT math. From modifying the original condition and the question, we can obtain a table below.
From the table, we can see that the question is a+b=? There are 4 variables (a,b,c, and d) and 2 equations (a+b+c+d=60 and a+c=38). In order to match the number of variables and the number of equations, we need 2 more equations. Since the condition 1) and 2) each has 1 equation, there is high chance that C is the answer.
Using both the condition 1) and 2), we get c=28. Also, since a=d, we get a+c=d+c=a+28=d+28=38. Then, a=d=10. So, if we substitute in a+b+c+d=60, we get 10+b+28+10=60. Then, b=12. So, a+b=10+12=22. So, the answer is unique and the condition is sufficient. So we can see how C could be the answer.
However, a question involving hidden integer is one of key questions (integer, statistics, inequality, probability, absolute value) and we have to consider Mistake Type 4(A).
In the case of the condition 2), since a=d, we get a+c=d+c=38. Then, if we substitute it into a+b+c+d=60, we get a+b+38=60. So, a+b=22. The answer is unique and the condition is sufficient. If both C and B are the answers to the question, the final correct answer choice is B.
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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