# Thread: percentiles and z scores

1. ## percentiles and z scores

I've already missed two points on this one..but I can't really figure out what I did wrong. Help would be appreciated.

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The scores on a standardized test are normally distributed with a mean of 50 and a standard deviation of 10. What is the percentage of scores that are greater than 75? Use the Standard Normal Table.

my work:

z = (x-mean)/standard dev.
z = (75-50)/10
z = 2.5

then, using the standard normal table... 2.5 give you .9938.

to percentage... 99.38%
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Where did I go wrong? thanks.

2. What you found was $p(z < 2.5)$, the percentage of scores that are less than 75.

What you're looking for is: $p(x > 75) = p(z > 2.5) = 1 - p(z < 1.5)$

3. ah... alright. So my answer would be .62%. Thanks, man.