# Spring 2013 – MAT 1272 Statistics – Reitz

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• Questions about WeBWorK, classwork, and exam reviews
• #16926

Can anyone please explain how to do problem #8 on the exam 3 review sheet???

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?

#16927

kalianne
Participant

The average time to finish the race is 3.25 hours.. So in order to beat 75% of the participants you would have to beat the average time.
The best thing for me to remember is when i see a percentage i know that i have to convert that to a probability ( a decimal) so i take 75 and divide that by a 100 which gives me 0.75 and i know that is to the right since i’m looking to beat the average race time and since its to the right you subtract 1
you then take that 1- .75= .25 and add two zero to create four decimal places which gives you .2500, look this number up in the chart. you wont always find that exact number so you find the one closest to it which is .2514 and that z- score is a -0.67

you the plug that -0.67 into the formula X= Mean+ Z(standard deviation) X= 3.25+ -0.67(0.5) which gives you 2.915 and then you round to 2.92

Hope this helps

#16941

Thanks Valerie, this really really helped a lot!!!! I great appreciate it :)

#16943

kalianne
Participant