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PSTAT 5A
Unit 4 Practice Problems
1. A bus arrives at a particular stop between 8:00am and 8:10am (assume the
bus is equally likely to arrive at any time within these 10 minutes). If Timmy
arrives at 8:00, what is the probability he will have to wait more than 8
minutes? …between 2.5 to 5 minutes? About how long, on average, will he
have to wait for the bus?
2. If Z follows the standard normal distribution, find the following probabilities.
A. P(z > -1.29)
B. P(0.24 < z < 2.14)
C. P(z ≥ 1.23)
D. P(z > 5.19)
3. Monthly utility bills in a certain city are normally distributed with mean $80
and standard deviation $12. A random bill is selected. Find the probability it
is…
A. …between $80 and $115.
B. …less than $50.
C. …more than $95.
4. IQ scores are approximately normally distributed with mean 100 and
variance 256. If Timmy scores in the top 1% of IQ scores, what is his actual
score? What is the raw score of a person who is in the 92nd percentile of IQ
scores?