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.
A certain city owns 298 buses, whose routes are divided into three zones. At a given time, if q buses are in the blue zone, r buses are in the red zone, s buses are in the green zone, and t buses are in garages, how many buses are in the green zone?
(1) The ratio of q to r to s is 4:6:7 and 26 buses are in garages.
(2) q = 64
Modify the original condition and the question. Since 298 buses are divided into three zones, it should be informed that how many t buses are in the blue zone, the red zone, and the green zone respectively. However, that information is in 1) & 2). Thus, the answer is E.
That is, in order to figure out variables, you need to know the number of q,r,s,t buses and t buses divided into each zone, which leads to a lot of variables. 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.