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February 27th, 2016, 06:16 PM  #1 
Newbie Joined: Feb 2016 From: Colorado Posts: 28 Thanks: 0  Stats practice question
I have no idea where to even begin The mean grade on a statistics quiz is 8 with a standard deviation of 0.5. What percent of the scores will be higher than 9? Assume the data is symmetrical. 
February 27th, 2016, 06:24 PM  #2 
Senior Member Joined: Dec 2012 From: Hong Kong Posts: 853 Thanks: 311 Math Focus: Stochastic processes, statistical inference, data mining, computational linguistics 
Can you assume the distribution is normal? If so, let X be the grade somebody gets on the test. $\displaystyle X \sim N(\mu = 8, \sigma = 0.5)$. $\displaystyle P(X > 9) = P(\frac{X  \mu}{\sigma} > \frac{9  8}{0.5}) = P(Z > 2) \approx 2.5\% $ (the exact answer will depend on the statistical table that your teacher gave you, but if you don't have a table, $\displaystyle 0.5  \frac{0.95}{2} = 0.025$ is what you should answer). Last edited by 123qwerty; February 27th, 2016 at 06:27 PM. Reason: Clarify 

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