As the mean is E[X] =150 and 
= 30, using the normal distribution to get the answer is just using Z. So, what we need is P(X<180).
![Z= \frac{x-E[X]}{\sigma } = \frac{180-150}{30} = \frac{30}{30} = 1](https://tex.z-dn.net/?f=Z%3D%20%20%5Cfrac%7Bx-E%5BX%5D%7D%7B%5Csigma%20%7D%20%3D%20%5Cfrac%7B180-150%7D%7B30%7D%20%3D%20%20%5Cfrac%7B30%7D%7B30%7D%20%3D%201)
P(x<180) = P(z< 1) =
(1) = 0.8413.
Then, just multiplying the amount of books you have, which is 500, with the probability would give how many books are less than 180 pages, being:
P(x<180) = P(z< 1) =
Then, just multiplying the amount of books you have, which is 500, with the probability would give how many books are less than 180 pages, being: