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/3 = 4/b , is a less than b?
(1) b ≥ 4
(2) b ≤ 5
There are 2 variables (a,b) and an equation (a/3=4/b), which means we need 1 more equation to be able to solve for 2 variables; the conditions provide us with 2 more equations, making (D) likely to be the answer.
Looking at condition 1, if b≥4, b=12/a≥4 and 12/4≥a, 3≥a. The answer is always ‘yes’ in this range, so it is sufficient.
For condition 2, the answer is ‘yes’ for b=4, a=3, but ‘no’ for b=-4, a=-3, so this condition is insufficient. Therefore, the answer becomes (A).
For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.