7. (ASV, Exercise 2.20 ) - 3 points. A fair die is rolled repeatedly. Use precise notation of probabilities of events and random variables for the solutions to the questions below. (a) Write down a precise sum expression for the probability that the first five rolls give a three at most two times. (b) Calculate the probability that the first three does not appear before the fifth roll. (c) Calculate the probability that the first three appears before the twentieth roll but not before the fifth roll.

0.9646 ; 0.5685

Step-by-step explanation:

Given that :

P(obtaining 3 on a die roll) = 1/6 = 0.1667

Obtaining 3 at most 2 times in five trials

Using binomial distribution formula :

P(x =x) = nCx * p^x * (1 - p)^(n - x)

n = 5 ;, p = 0.1667

P(x ≤ 2) = P(x = 0) + P(x =1) + P(x = 2)

Using the binomial probability :

P(x ≤ 2) = 0.4019 + 0.4019 + 0.1608

P(x ≤ 2) = 0.9646

B.) using geometric distribution :

(1 - p)^x-1 * p

P ≥ 5) = 1 - P(x ≤ 4) = 1 - p(x = 0) + p(x = 1) + p(x =2) + p(x = 3)

1 - (0.1667 + 0.1157+0.0965+0.0804)

= 0.5685