Question:

In answering a question on a multiple choice test , an examine either knows the answer or he guesses or he copies. Suppose each question has 4 choices. Let the probability that examine copies the answer is 1/6 and the probability that he guesses is 1/3. The probability that his answer is correct given that he copied the answer is 1/8. Suppose an examine answers a question correctly, what is the probability that he really knows the answer

What approch should i take to start solving this problem . Should i use the Bayes theorem or no? please provide with the name of the procedure / theorem