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.
If a certain company purchased computers at $2000 each and printers $300 each , how many computers did it purchase?
(1) More than three printers were purchased
(2) The total amount for the purchase of the computers and printers was $15,000
When you modify the original condition and the question, it is frequently given on GMAT Math test which is "2 by 2" que.
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On the table, there are 2 variables(a,b), which should match with the number of equations. So you need 2 equations. For 1) 1 equation, for 2) 1 equation, which is likely to make C the answer. When 1) & 2), 2000a+200b=15000 -> 20a+3b=150 -> (a,b)=(3,30),(6,10), which is not unique and not sufficient. Therefore, the answer is E.
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.