When I saw these questions, I always felt like closing my eyes and hope they'd go away, but unfortunately they remained. With all due seriousness, I tell you these questions are not as hard as they may appear. It sounds like you've done your studying of critical path so I'm confident you know what to do. You really want to know if there's a shorthand version to get to the answer and in my opinion, I recommend you continue with the drawing because it accomplishes two important steps.
One, it visualizes the arrangement of events you need to determine the critical path. Now in this case you answered incorrectly and that should not make you worry. You may have simply miscounted somewhere and that is easily overcome with just a bit more analysis of the question.
Two, by drawing the diagram yourself, you will be prepared for further questions that may be later during the test, which might ask about a different aspect of the critical path or how changes could affect timelines.
The result of these steps will reinforce your comprehension of the critical path concept and make you faster and better come exam time.