Johnathan has a history test. There are a total of 6 essay prompts, but Jonathan only studies two of the prompts the night before the test. On the test day, the professor rolls a standard die three times to select three random essay questions. What is the probability that:
a. one essay Jonathan studied for is randomly chosen?
b. both essays Jonathan studied for are randomly chosen?
Total number of essays = 6
Number of essays Jonathan studied = 2
Number of essays Jonathan did not study = 6 - 2 = 4
Number of ways to select r items from n, nCr = n!/(r! x (n-r)!)
a) P(one essay Jonathan studied for is randomly chosen) = Number of ways to chose 1 essay from the 2 Jonathan studied x Number of ways to chose 2 essays from the 4 Jonathan did not study / Total number of ways to chose 3 essays from total 6
= 2C1 x 4C2 / 6C3
= 0.6
b) P(both essays Jonathan studied for are randomly chosen) = Number of ways to chose 2 essays from the 2 Jonathan studied x Number of ways to chose 1 essays from the 4 Jonathan did not study / Total number of ways to chose 3 essays from total 6
= 2C2 x 4C1 / 6C3
= 0.2
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