In: Statistics and Probability
A group of 20 students contains 10 who believe Anna is doing a good job, 5 who believe Anna is doing a bad job and 5 who do not know who Anna is. Two students are chosen at random. What are the probabilities? Both do not know who Anna is One believes Anna is doing a good job and 1 who believes Trump is doing a poor job
Number of ways to select two students from a group of 20 students = 20C2 = 20! / ((20-2)! 2!) = (20 * 19)/2 = 190
Number of ways to select two students who do not know who Anna is = 5C2 = 5! / ((5-2)! 2!) = (5 * 4)/2 = 10
Probability that both students do not know who Anna = Number of ways to select two students who do not know who Anna is / Number of ways to select two students from a group of 20 students
= 10 / 190 = 1/19
Number of ways to select two students in which one believes Anna is doing a good job and 1 who believes Anna is doing a poor job = 10C1 * 5C1 = 10 * 5 = 50
Probability that one believes Anna is doing a good job and 1 who believes Anna is doing a poor job = Number of ways to select two students in which one believes Anna is doing a good job and 1 who believes Anna is doing a poor job / Number of ways to select two students from a group of 20 students
= 50 / 190
= 5 / 19