Answer:
(1) The probability that the technician tests at least 5 computers before the 1st defective computer is 0.078.
(2) The probability at least 5 computers are infected is 0.949.
Step-by-step explanation:
The probability that a computer is defective is, <em>p</em> = 0.40.
(1)
Let <em>X</em> = number of computers to be tested before the 1st defect is found.
Then the random variable
.
The probability function of a Geometric distribution for <em>k</em> failures before the 1st success is:

Compute the probability that the technician tests at least 5 computers before the 1st defective computer is found as follows:
P (X ≥ 5) = 1 - P (X < 5)
= 1 - [P (X = 0) + P (X = 1) + P (X = 2) + P (X = 3) + P (X = 4)]
![=1 -[(1-0.40)^{0}0.40+(1-0.40)^{1}0.40+(1-0.40)^{2}0.40\\+(1-0.40)^{3}0.40+(1-0.40)^{4}0.40]\\=1-[0.40+0.24+0.144+0.0864+0.05184]\\=0.07776\\\approx0.078](https://tex.z-dn.net/?f=%3D1%20-%5B%281-0.40%29%5E%7B0%7D0.40%2B%281-0.40%29%5E%7B1%7D0.40%2B%281-0.40%29%5E%7B2%7D0.40%5C%5C%2B%281-0.40%29%5E%7B3%7D0.40%2B%281-0.40%29%5E%7B4%7D0.40%5D%5C%5C%3D1-%5B0.40%2B0.24%2B0.144%2B0.0864%2B0.05184%5D%5C%5C%3D0.07776%5C%5C%5Capprox0.078)
Thus, the probability that the technician tests at least 5 computers before the 1st defective computer is 0.078.
(2)
Let <em>Y</em> = number of computers infected.
The number of computers in the company is, <em>n</em> = 20.
Then the random variable
.
The probability function of a binomial distribution is:

Compute the probability at least 5 computers are infected as follows:
P (Y ≥ 5) = 1 - P (Y < 5)
= 1 - [P (Y = 0) + P (Y = 1) + P (Y = 2) + P (Y = 3) + P (Y = 4)] ![=1-[{20\choose 0}(0.40)^{0}(1-0.40)^{20-0}+{20\choose 1}(0.40)^{1}(1-0.40)^{20-1}\\+{20\choose 2}(0.40)^{2}(1-0.40)^{20-2}+{20\choose 3}(0.40)^{3}(1-0.40)^{20-3}\\+{20\choose 4}(0.40)^{4}(1-0.40)^{20-4}]\\=1-[0.00004+0.00049+0.00309+0.01235+0.03499]\\=1-0.05096\\=0.94904](https://tex.z-dn.net/?f=%3D1-%5B%7B20%5Cchoose%200%7D%280.40%29%5E%7B0%7D%281-0.40%29%5E%7B20-0%7D%2B%7B20%5Cchoose%201%7D%280.40%29%5E%7B1%7D%281-0.40%29%5E%7B20-1%7D%5C%5C%2B%7B20%5Cchoose%202%7D%280.40%29%5E%7B2%7D%281-0.40%29%5E%7B20-2%7D%2B%7B20%5Cchoose%203%7D%280.40%29%5E%7B3%7D%281-0.40%29%5E%7B20-3%7D%5C%5C%2B%7B20%5Cchoose%204%7D%280.40%29%5E%7B4%7D%281-0.40%29%5E%7B20-4%7D%5D%5C%5C%3D1-%5B0.00004%2B0.00049%2B0.00309%2B0.01235%2B0.03499%5D%5C%5C%3D1-0.05096%5C%5C%3D0.94904)
Thus, the probability at least 5 computers are infected is 0.949.