Answer:
a) Probability mass function of x
x P(X=x)
0 0.0602
1 0.0908
2 0.1700
3 0.2050
4 0.1800
5 0.1550
6 0.0843
7 0.0390
8 0.0147
b) Cumulative Distribution function of X
x F(x)
0 0.0602
1 0.1510
2 0.3210
3 0.5260
4 0.7060
5 0.8610
6 0.9453
7 0.9843
8 1.0000
The cumulative distribution function gives 1.0000 as it should.
Explanation:
Probability of arriving late = 0.43
Probability of coming late = 0.57
Let's start with the probability P(X=0) that exactly 0 people arrive late, the probability P(X=1) that exactly 1 person arrives late, the probability P(X=2) that exactly 2 people arrive late, and so on up to the probability P(X=8) that 8 people arrive late.
Interpretation(s) of P(X=0)
The two singles must arrive on time, and the three couples also must. It follows that P(X=0) = (0.57)⁵ = 0.0602
Interpretation(s) of P(X=1)
Exactly 1 person, a single, must arrive late, and all the rest must arrive on time. The late single can be chosen in 2 ways. The probabiliy that (s)he arrives late is 0.43.
The probability that the other single and the three couples arrive on time is (0.57)⁴
It follows that
P(X=1) = (2)(0.43)(0.57)⁴ = 0.0908
Interpretation(s) of P(X=2)
Two late can happen in two different ways. Either (i) the two singles are late, and the couples are on time or (ii) the singles are on time but one couple is late.
(i) The probability that the two singles are late, but the couples are not is (0.43)²(0.57)³
(ii) The probability that the two singles are on time is (0.57)²
Given that the singles are on time, the late couple can be chosen in 3 ways. The probability that it is late is 0.43 and the probability the other two couples are on time is (0.57)².
So the probability of (ii) is (0.57)²(3)(0.43)(0.57)² which looks better as (3)(0.43)(0.57)⁴ It follows that
P(X=2) = (0.43)²(0.57)³ + (3)(0.43)(0.57)⁴ = 0.0342 + 0.136 = 0.1700
Interpretations of P(X=3).
Here a single must arrive late, and also a couple. The late single can be chosen in 2 ways. The probability the person is late but the other single is not is (0.43)(0.57).
The late couple can be chosen in 3 ways. The probability one couple is late and the other two couples are not is (0.43)(0.57)². Putting things together, we find that
P(X=3) = (2)(3)(0.43)²(0.57)³ = 0.2050
Interpretation(s) P(X=4)
Since we either (i) have the two singles and one couple late, or (ii) two couples late. So the calculation will break up into two cases.
(i) Two singles and one couple late
Two singles' probability of being late = (0.43)² and One couple being late can be done in 3 ways, so its probability = 3(0.43)(0.57)²
(ii) Two couples late, one couple and two singles early
This can be done in only 3 ways, and its probability is 2(0.57)³(0.43)²
P(X=4) = (3)(0.43)³(0.57)² + (3)(0.57)³(0.43)² = 0.0775 + 0.103 = 0.1800
Interpretations of P(X=5)
For 5 people to be late, it has to be two couples and 1 single person.
For couples, The two late couples can be picked in 3 ways. Probability is 3(0.43)²(0.57)
The late single person can be picked in two ways too, 2(0.43)(0.57)
P(X=5) = 2(3)(0.43)³(0.57)² = 0.1550
Interpretations of P(X=6)
For 6 people to be late, we have either (i) the three couples are late or (ii) two couples and the two singles.
(i) Three couples late with two singles on time = (0.43)³(0.57)²
(ii) Two couples and two singles late
Two couples can be selected in 3 ways, so probability = 3(0.43)²(0.57)(0.43)²
P(X=6) = (0.43)³(0.57)² + 3(0.43)⁴(0.57) = 0.0258 + 0.0585 = 0.0843
Interpretation(s) of P(X=7)
For 7 people to be late, it has to be all three couples and only one single (which can be picked in two ways)
P(X=7) = 2(0.57)(0.43)⁴ = 0.0390
Interpretations of P(X=8)
Everybody had to be late
P(X=8) = (0.43)⁵ = 0.0147
