Question

Suppose over several years of offering AP Statistics, a high school finds that final exam
scores are normally distributed with a mean of 78 and a standard deviation of 6.
mean=78 sd=1.107
What's the probability a sample of scores will have a mean greater than 80?
I did it on my calculator and got 0.0092435904 just want to make sure I did it right any feedback would be great :)

Answer 1

How did you get sd=1.107?

Answer 2

sd (6)/square root of 50. Forgot to add n=50 @fibonaccichick666

Answer 3

ah, ty. Lemme see.

Answer 4

okay :) @fibonaccichick666

Answer 5

so for your z-score you got 2/1.107?

Answer 6

cause i'm getting like 4% probability

Answer 7

same

Answer 8

i think i mixed up my problems the sd should be 0.849 and i put it on my calculator by doing normalcdf(80,E99,78,0.849) did I do it wrong then?

Answer 9

uhm, I don't know the calculator commands. I just use a table, but I did catch that mistake now that you mentioned it. so sd=0.84852813742 that makes us get z=80-78/0.84852813742 which is 2.35702260397

Answer 10

based off of that z-score of about 2.35 or 2.36, we get somewhere between 1-.9906 and 1-.9909 for the probability

Answer 11

So now that we have the right values, your answer seems reasonable to me

Answer 12

so would 0.0092435904 be right then? @FibonacciChick666

Answer 13

I'd write it as a percent, but I find it believable. By my chart I get .00925 when I average the values

Answer 14

Those are close enough for me to believe you are correct

Answer 15

okay thank you! i thought I was doing the whole lesson wrong, but you just made me think differently :D

Answer 16

oh, np, best of luck! I personally like the charts better because I can't remember all the calculator command crap. (unless I have my ti-89 handy and it tells me what to put in)