Question

1. (a) The life time of a certain brand of bulbs produced by a company is normally distributed, with mean 210 hours and standard deviation 56 hours. What is the probability that a bulb picked at random from this company’s products will have a life time of:

(i) at least 300 hours,
(ii) at most 100 hours,
(iii) between 150 and 250 hours.
(b) In a contest, two friends, Kofi and Mensah were asked to solve a problem. The
probability that Kofi will solve it correctly is ' and the probability that Mensah
(
will solve it correctly is ) . Find the probability that neither of them solved it correctly.
*

Answer 1

1a) (i) 0.0537

(ii) 0.0250

(iii) 0.6188

1b) The probability that neither Kofi nor Mensah solves the problem correctly = (9/20) = 0.45

Explanation:

The complete Question is presented in the attached image to this answer.

1a) This is a normal distribution problem with

Mean lifetime of bulbs = μ = 210 hours

Standard deviation = σ = 56 hours

(i) at least 300 hours, P(x ≥ 300)

We first standardize 300 hours

The standardized score for any value is the value minus the mean then divided by the standard deviation.

z = (x - μ)/σ = (300 - 210)/56 = 1.61

To determine the required probability

P(x ≥ 300) = P(z ≥ 1.61)

We'll use data from the normal probability table for these probabilities

P(x ≥ 300) = P(z ≥ 1.61) = 1 - P(z < 1.61)

= 1 - 0.94630 = 0.0537

(ii) at most 100 hours, P(x ≤ 100)

We first standardize 100 hours

z = (x - μ)/σ = (100 - 210)/56 = -1.96

To determine the required probability

P(x ≤ 100) = P(z ≤ -1.96)

We'll use data from the normal probability table for these probabilities

P(x ≤ 100) = P(z ≤ -1.96) = 0.0250

(iii) between 150 and 250 hours.

P(150 < x < 250)

We first standardize 150 and 250 hours

For 150 hours

z = (x - μ)/σ = (150 - 210)/56 = -1.07

For 250 hours

z = (x - μ)/σ = (250 - 210)/56 = 0.71

To determined the required probability

P(150 < x < 250) = P(-1.07 < z < 0.71)

We'll use data from the normal probability table for these probabilities

P(150 < x < 250) = P(-1.07 < z < 0.71)

= P(z < 0.71) - P(z < -1.07)

= 0.76115 - 0.14231

= 0.61884 = 0.6188 to 4 d.p.

1b) Probability that Kofi solves the problem correctly = P(K) = (1/4)

Probability that Mensah solves the problem correctly = P(M) = (2/5)

Probability that Kofi does NOT solve the problem correctly = P(K') = 1 - P(K) = 1 - (1/4) = (3/4)

Probability that Mensah does NOT solve the problem correctly = P(M') = 1 - P(M) = 1 - (2/5) = (3/5)

To find the probability that neither of them solves the problem correctly, we first make the logical assumption that the probabilities of either of them solving the problem are independent of each other.

Hence, the probability that neither of them solves the problem correctly = P(K' n M')

P(K' n M') = P(K') × P(M') = (3/4) × (3/5) = (9/20) = 0.45

Hope this Helps!!!