Normal distribution approximation

A multiple choice test consists of 50 questions and each question has four answers from which to choose. If a student guesses every answer, what is the probabilty that the student

b) will get more than 10 questions correct?

here is what i did. I got close, so it could just be the book??

p=1/4 = .25 np=(.25)(50) = 12.5 therefore np is greater than 5

q=3/4=.75 nq=(.75)(50)= 37.5 therefore nq is great than 5

Since np and nq are both greater than five we can use the formula Standard deviation =squareroot(npq) (sorry i don't know the computer way to do that)

standard deviation = root((50)(.25)(.75))

=3.061862178

p(11 or more) because you want to know the probablitiy of getting more than 10 questions correct so 10 is not included

**np=mean=12.5 ** this is the thing im not sure about .. but i found it in my notes so i went with it.

z=(11-12.5)/3.061862178

=-0.49 area to the left = 0.3121

we want the area to the right

1-0.3121 = 0.6879

so 68.79%

the answer in the back of the book is 74.32%