Suppose that a principal of a local high school tracks the number of minutes his students spend texting on a given school day. He finds that the distribution of minutes spent texting is roughly normal with a mean of 60 and a standard deviation of 20.

Question

a. the percentage of students who texted for fewer than 50 minutes.
b. the probability of selecting at random a student who spends between 10 and 30 minutes texting.
c. the probability of selecting at random a student who spends an extreme amount of time texting – either less than 10 minutes OR more than 110 minutes.
d. the probability of selecting at random (with replacement) two students who spent a below-average amount of time texting.
e. the probability of selecting at random (with replacement) two students who spent more than 75 minutes texting.
f. the percentile rank of a student who spent 100 minutes texting.
the two numbers of minutes that define the middle 95% of students in the distribution

Answer 1

Z = (score-mean)/SD

Use table labeled area under normal distribution in the back of your text.

Either-or probabilities found by adding individual probabilities.

Both probabilities found by multiplying individual probabilities.