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Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
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The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.
The inequality bx - (4a + b) < 0 is equivalent to bx < (4a + b).
Condition 1)
Since a < 0 and a = - b, we have b = -a and b is negative.
bx < (4a + b) is equivalent to x > (4a + b)/b, (the direction of the inequality sign changes because we are dividing by a negative number). Then x > (-4b + b)/b (by replacing a with -b), x > (-3b)/b, or x > -3.
The solution is x > -3, so we get yes as the answer.
Since condition 1) yields a unique solution 'yes', it is sufficient.
Condition 2)
The inequality ax - (a - 2b) > 0 is equivalent to ax > (a - 2b).
Since its solution set is x < 3, we have x < (a - 2b)/a = 3. So, 3a = a - 2b or a = -b and a is negative. It is same as condition 1).
Since condition 2) yields a unique solution, it is sufficient.
Therefore, D is the answer.
Answer: D
Note: Tip 1) of the VA method states that D is most likely the answer if condition 1) gives the same information as condition 2).
Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
