Could someone check to see if I have these correct?

Question

A national placement test has a normal distribution with a mean of 500 and a standard deviation of 100.

a) Find the z-score of 360. (-1.4)

b) Find the probability that a randomly selected student scores less than 640. (0.9192)

c) Find the probability that a randomly selected student scores more than 420. (0.2119)

d) Find the probability that a randomly selected student scores between 400 and 650. (0.7073)

e) Find the 90th percentile of the distribution (630)

Answer 1

a and b are correct

C is wrong. You want 1-(0.2119) = 0.7881
because you're asked for the area to the right of 420. Think about it: probabilites are 1/2 at the mean (500) so the probabilty of scoring higher than 420 must be greater than 1/2.

D: I used my TI-83 and got 0.7745.

E: Looks good. I used my TI-83 to find the probablity of your number 630 and go p=0.9032. That's good agreement.