Question

The time for a professor to grade an exam is normally distributed with a mean of 16.3 minutes and a standard deviation of 4.2 minutes. What is the probability that a randomly selected exam will require less than 20 minutes to grade

Answer 1

Answer:

81.06% probability that a randomly selected exam will require less than 20 minutes to grade

Step-by-step explanation:

Problems of normally distributed samples are solved using the z-score formula.

In a set with mean $\mu$ and standard deviation $\sigma$, the zscore of a measure X is given by:

$Z = \frac{X - \mu}{\sigma}$

The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.

In this problem, we have that:

$\mu = 16.3, \sigma = 4.2$

What is the probability that a randomly selected exam will require less than 20 minutes to grade

This is the pvalue of Z when X = 20.

$Z = \frac{X - \mu}{\sigma}$

$Z = \frac{20 - 16.3}{4.2}$

$Z = 0.88$

$Z = 0.88$ has a pvalue of 0.8106

81.06% probability that a randomly selected exam will require less than 20 minutes to grade