Normal curve, be in top 25%?

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A psychologist has been studying eye fatigue using a particular measure, which she administers to students after they have worked for 1 hour writing on a computer. On this measure, she has found that the distribution follows a normal curve. The test of eye fatigue has a mean of 15 and a standard deviation of 5. Using a normal curve table, what score on the eye fatigue measure would a person need to have to be in the top 40%? 

Oct 13th, 2015

Here is my solution.

http://www.mathsisfun.com/data/standard-normal-distribution-table.html This is the standard deviation table I used.  

On the table you have to find what z-score has a 0.60 area meaning its in the top 40% because 60% of the curve is below it. This is going to give a z-score of about .26 (the area for this is 0.6026).  The formula for z-score is (value-mean)/standard deviation.  z=(v-m)/s We can rearrange this to zs=v-m then zs+m = v

Plugging in values we get zs+m=v, (0.26)(5)+15=16.3

If you want want a calculator would tell you I can redo  the math because it will have a more accurate z-score.

Don't be afraid to ask me any more questions. Please ask me for clarification before you withdraw.
Oct 13th, 2015

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