Question

Before a computer is assembled, its motherboard goes through a special inspection. Assume only 85% of motherboards pass this inspection.
(a) What is the probability that at least 13 of the next 15 motherboards pass inspection?
(b) On the average, how many motherboards should be inspected until a motherboard that passes inspection is found?

Answer 1

Answer:

(a) The probability that at least 13 of the next 15 motherboards pass inspection is 0.6042.

(b) On average, 1.18 motherboards should be inspected until a motherboard that passes inspection is found.

Step-by-step explanation:

Let X = number of motherboards that pass the inspection.

The probability that a motherboards pass the inspection is P (X) = p = 0.85.

The random variable X follows a Binomial distribution with parameters n and p.

The probability mass function of X is,

$P(X=x)={n\choose x}p^{x}(1-p)^{n-x};\ x=0,1,2,3...$

(a)

Compute the probability that at least 13 of the next 15 motherboards pass inspection as follows:

P (X ≥ 13) = P (X=  13) + P (X = 14) + P (x = 15)

               $={15\choose 13}0.85^{13}(1-0.85)^{15-13}+{15\choose 13}0.85^{14}(1-0.85)^{15-14}\\+{15\choose 13}0.85^{16}(1-0.85)^{15-15}\\=0.2856+0.2312+0.0874\\=0.6042$

Thus, the probability that at least 13 of the next 15 motherboards pass inspection is 0.6042.

(b)

Computed the expected number of motherboards should be inspected until a motherboard that passes inspection is found as follows:

$Expected\ value=\frac{1}{p}=\frac{1}{0.85}= 1.1764\approx1.18$

Thus, on average, 1.18 motherboards should be inspected until a motherboard that passes inspection is found.