Nour,
The "correct" formula will be dependent on what kind of information you're given in the question (most common) or what kind of information you can derive from the information you have in the question (less common). In general, you will not have to go though multiple steps to arrive at the answer, but I've seen a few in the Simulator that are two-step problems. On my ancient version of the PMP exam, all of my "math" questions were one-step.
The important thing is that you know the relationships between all of the various terms and how they are derived.
Harry