The question stem specifies that abc ≠ 0. In other words, none of the three variables is equal to 0. It also says that a = 3bc and asks for the value of c, specifically. Given that equation, what would you need to know in order to find c? Rearrange it to get c by itself on one side:
a = 3bc
a / 3b = c
If you know the values of a and b individually, then you can find c. But if you know the value of a / b, then you can plug that into the equation as well to find c. This is a Combo question: The combination of variables a / b is also sufficient to answer the question.
(1) INSUFFICIENT: You can't find the individual values of a and b from this one equation. Rearrange the given equation to try to find the combo:
a = 10 – b
a – b = 10
This is a combination, but not the right combination; you need the Combo a / b. This statement isn't sufficient to answer the question.
(2) SUFFICIENT: You can't find the individual values of a and b from this one equation. Rearrange the given equation to try to find the combo:
3a = 4b
a / b = 4 / 3
This value can be substituted into the equation from the question stem to find c. (Don't actually calculate c. On Data Sufficiency, stop when you know that you can calculate the value.)
The correct answer is (B): Statement (2) by itself is sufficient to answer the question.
