How many people in a group of 50 own neither a fax machine nor a laser

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.

How many people in a group of 50 own neither a fax machine nor a laser printer?

(1) The total number of people in the group who own a fax machine or a laser printer or both is less than 50.
(2) The total number of people in the group who own both a fax machine and a laser printer is 15.

When it comes to a question like the above, the question is frequently given in the Gmat Math test, which is "2 by 2" que like the table below.

When you look at the table, there are 4 variables(a,b,c,d) and 1 equation(a+b+c+d=50), which should match with the number of equations. So, you need 3 more equations. For 1) 1 equation, For 2) 1 equation, which is likely to make E the answer. In 1) & 2), it is a+b+c<50, a=15 and the value of d is not unique, which is not sufficient. Therefore, the answer is E.

-> For cases where we need 3 more equations, such as original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 80% chance that E is the answer (especially about 90% of 2 by 2 questions where there are more than 3 variables), while C has 15% 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 E 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, C or D.