# I need help with standard deviation word problems?

• April 10th 2011, 06:47 PM
Konklinm
I need help with standard deviation word problems?
Haha sorry if my title isn't accurate, I'm not really sure what to call these.
I was absent two days in math class and my ***** of a teacher refuses to give me any sort of help even though my absences are excused. Now i'm stuck with two days of lessons to learn on my own :(

I have two word problems that I have NO idea how to do. Can someone please explain them to me? I don't just want answers, I really need someone to tell me what to do to solve them.

questions 1:
"William made a 78 on an exam where the class grades were normally ditrubited with an average of 86 and a standard deviation of 4. Which is true about William's score?
a)His score is above the 50th precentile of the class
b)His score is below the 2nd percentile of the class
c)His score was 2 standard deviations below the mean
d)His score was 2 standard deviations above the mean"

question 2:
"Lewis earned an 84 on his biology midterm and an 82 on his history midterm. In Biology, the class mean was 79 with a standard deviations of 5. In history, the class mean was 76 with a standard deviation of 3.
a) for each midterm, how many standard deviations is Lewis from the mean?
b) On wich test did he do better compared to the rest of the class?"

Thank you so much! I really appreciate your help :)
• April 10th 2011, 07:06 PM
pickslides
For question 1 I like c) as the answer. But why would I choose that?

for the second one (and the first one for that matter) consider $\displaystyle z=\frac{x-\mu}{\sigma}$