If scores are normally distributed with a mean of 35 and a standard deviation of 10, what percent of the scores is: (a) greater than 34? (b) smaller than 42? (c) between 28 and 34?

MATLAB: An Introduction with Applications
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Author:Amos Gilat
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Answer the following questions:

  • If scores are normally distributed with a mean of 35 and a standard deviation of 10, what percent of the scores is: (a) greater than 34? (b) smaller than 42? (c) between 28 and 34? 
  • A test is normally distributed with a mean of 70 and a standard deviation of 8. (a) What score would be needed to be in the 85th percentile? (b) What score would be needed to be in the 22nd percentile?
  • A normal distribution has a mean of 20 and a standard deviation of 4. Find the Z scores for the following numbers: (a) 28 (b) 18 (c) 10 (d) 23.
  • Assume the speed of vehicles along a stretch of I-10 has an approximately normal distribution with a mean of 71 mph and a standard deviation of 8 mph. The current speed limit is 65 mph. What is the proportion of vehicles less than or equal to the speed limit?
  • A group of students at a school takes a history test. The distribution is normal with a mean of 25, and a standard deviation of 4. (a) Everyone who scores in the top 30% of the distribution gets a certificate. What is the lowest score someone can get and still earn a certificate? 
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for a) what is happening here?

PX>34=Px-μσ>34-3510=Pz>-0.1=0.5398

to get z if we plug in our given numbers, i end up with -316. I am not sure where the -0.1 comes in and where we get the 0.5398

I did z= 34- 35(10)

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