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 rectangular boxes with the dimensions 1 foot by 3 feet by 10 feet can be placed into a rectangular carton that has a volume of v cubic feet?
(1) The carton is a cube.
(2) v = 300
There are 3 variables (length, width, height) and 2 equations are given from the 2 conditions, so there is high chance (E) will be our answer.
Looking at the conditions together ,
if we let the length of one side of the carton be x, x^3=300, x=6.7. We cannot put in even one box, so the conditions are sufficient and the answer becomes (C).
For cases where we need 3 more equation, 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.