Suppose that a sample of 50 scores has a mean of Y=90 and the standard deviation is 15.
Between what two values of Y will at least 90% of the scores fall?
What is the maximun number of scores that can lie out of the interval?
90% = mean ± 1.645SD
If 90% fall within that interval, what percent would you expect to fall outside? How many scores would that be?