Study Guide for Problem #8 Exam 3 review

Jacky Xu

8. The finishing times for a long-distance race are normally distributed, with an average finishing time of 3.25 hours and a standard deviation of 0.5 hours. If bob is running this race, what time does he need to finish in order to beat 75% of the other participants?

Step 1: You first have to write down the variables that is mentioned in the problem.
Mean: 3.25
Standard Deviation: 0.5

Step 2: Since you are trying to find how much time is needed to beat 75% of the other participants, you have to find the right side of the graph rather then the left side, which makes it so you have to find the closest Z-score in the Standard Normal Distribution chart for 25% because he needs to be on the other 25% to beat the other 75%, which makes it so you have to find 0.25 in the chart.
Z-score: -0.67

Step 3: After finding the Z-score, you can plug in the numbers by using the formula to convert Z to X by doing x = µ + z(σ) which then turns out to be X = 3.25 + -0.67(0.5)
X = 3.25 + -.335
X = 2.915

Step 4: The number you turn out with is X = 2.915, which means that the amount of time Bob needs to finish the race beating 75% of the people is 2.915.


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