Data Sufficiency

In the coordinate plane p≠1, a line L passes through a point (1,p). What is the slope of the line L?


1) The line L passes through point (0,1).
2) The line L passes through point (p,13).


*A solution will be posted in two days.

In the coordinate plane p1, a line L passes through a point (1,p). What is the slope of the line L?

1) The line L passes through point (0,1).
2) The line L passes through point (p,13).


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 the original condition, there are 3 variables(1,p),(x1,y1), which should match with the number of equations. So you need 3 equations. For 1) 1 equation, for 2) 1 equation, which is likely to make E the answer. When 1) & 2), (1,p),(0,1),(p,13) → (p-1/1-0)=(13-1/p-0) → p-1=12/p, p^2-p-12=0. From (p-4)(p+3)=0, p=-3,4. Then the slope is 3,-4, which means there are 2 answers. So it is not unique and 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.