I need help with this question as I am not getting the same answer as the back of the book

A high school operates 450 computers.

On a given day, an average of 15 of these

computers are out of service. A computer lab

contains 30 computers. A teacher would like

to use the lab with her class of 27 students.

a) Determine the mean and standard

deviation of the normal approximation.

b) Use the normal approximation to

determine the probability that there will

be enough working computers.

The answer to part a is: mean: 29.01, standard deviation: about 0.9463.

and to part d is: about 0.9383