13. A psychologist determined that thenumber of sessions required to obtain the trust of a new patient iseither 1,2, or 3. Let x be a random variable indicating thenumber of session required to gain the patients trust. Thefollowing probability function has been proposed.

f(x) =x/6 for x = 1, 2, or 3

a. is this probability function valid? Explain

b. what is the probability that takes exactly 2 sessionsto gain the patients trust

c. what is the probability that it takes at least 2 sessionsto gain the patients trust

51. Of the 25 students (14 boys and 11 girls) in the 6thgrade class at St. Andrews school, 5 students were absentthursday.

a. what is the probability that 2 of the absent studentswere girls?

b. what is the probability that 2 of theabsent students were boys?

c. what is the probability that all the absentstudents were boys?

d. what is the probability that none of theabsent students were boys?

c. what is the probability that all the absentstudents were boys?

d. what is the probability that none of theabsent students were boys?

## Answers ( 1 )

3 absent boysstudents can be selected from 14 boys in ways

probability that 2 of absentgirls is *

Similarly

probability that 2 of the absentees were boysis

probability that all the absentees were boys is

probability that none of the absent astudents were boys is