Mrs. Hudson plans to give a 10 question multiple choice exam in which each question has 4 options. Each question counts for 1 point. A score of 8 or more is a passing score while 7 or less is a failing score. The probabilities of guessing a certain number of questions correctly are listed in the table below. Number of Questions Correct
Probability for Guessing Correctly
0
0.05631351
1
0.18771172
2
0.28156757
3
0.25028229
4
0.14599800
5
0.05839920
6
0.01622200
7
0.00308990
8
0.00038624
9
0.00002861
10
0.00000095
Compute the probability of answering 8 or more questions correctly by calculating the probability with a direct sum and by using complements.
How do the two approaches compare?