Question

A manufacturer knows that their items have a normally distributed lifespan, with a mean of 5 years, and standard deviation of 1.3 years.

If you randomly purchase one item, what is the probability it will last longer than 6 years?

Answer 1

Explanation:

Let X be the lifespan of an item. We are given that X is normally distributed with a mean of μ = 5 years and a standard deviation of σ = 1.3 years.

We want to find the probability that an item will last longer than 6 years. Let Y be the random variable that represents the lifespan of an item in excess of 6 years, i.e. Y = X - 6. Then we want to find:

P(Y > 0)

Using the properties of normal distribution, we can standardize Y to get a standard normal variable Z:

Z = (Y - μ) / σ = (X - 6 - 5) / 1.3 = (X - 11) / 1.3

So we want to find:

P(Z > (6 - 11) / 1.3) = P(Z > -3.85)

Using a standard normal distribution table or calculator, we can find that the probability of Z being greater than -3.85 is very close to 1 (in fact, it is essentially 1). Therefore, the probability of an item lasting longer than 6 years is essentially the same as the probability of Y being greater than 0, which is 1.

Therefore, the probability that a randomly purchased item will last longer than 6 years is approximately 1.