2 people chosen from 80 (40 couples): find prob. married

[coolcep1]

need help please!
An audience consists of 40 married couples (80 total people). Two people are selected at random and without replacement from the audience. What is the probablility that two people chosen will be married to each other?
my work: 1- 78/80 = 1/40
the answer is 1/79 and i have no idea how to get there.

You are offered a sales job. You have the option of taking a fixed salary of $450 per week or a salary based on commission of $150 per sale. The manager tells you that you can expect to see 15 clients per week. He further tells you that the average rate of sales for his office is 20%. If you think that you can do as well as the office average, what is the probability that you will make at least as much working on commission as you would working on the fixed salary rate?
the answer is 0.602, i have no idea how to set up the problem.

 

[Loren]

Re: probablility

After selecting the first person randomly, how many people are there from which to choose the second person? In that group, how many are the first person's spouse?

 

[coolcep1]

Re: probablility

there would be 79/80 people after picking the first one and then 1/79 would be who could be married to that person picked??

 

[arthur ohlsten]

Re: probablility

probability of 0 sales= Q^15
Probability of $0=Q^15
P[0]=.035

prob. of 1 sale=15Q^14P
P[1]=.13
Probability of $ 150 =.13

prob of 2 sales= {15[14]/2}Q^13P^2
P[2]=.23
probability of earning $300 = .23

prob. of 3 sales = {[15][14][13]/3!}Q^12P^3
P[3]=.25
probability of earning $450=.25

expected gain of at least 3 sales = [.13][150]+[.23][300]+[.25][450]
Expected gain of 3 sales = $201

probability of at least 3 sales = .13+.23+.25=.61

do it more accurately for the answer given
Arthur