Hi Larry,
I think of the tree in three layers. First you answer the first question and get it correct (C) or incorrect (I), next you answer the second question and get it correct (C) or incorrect (I) and finally you answer the third question and get it correct (C) or incorrect (I). Here is my tree.
 Question one is true or false and the person is guessing, so the probability he guesses the correct answer is 1/2.
 Question two is true or false and the person is guessing, so the probability he guesses the correct answer is 1/2.
 Question three is true or false and the person is guessing, so the probability he guesses the correct answer is 1/2.
Thus the probability that he gets the first question correct and the second question correct and the third question correct is
1/2 1/2 1/2 = 1/8.
Penny
