The statement says that Kristen read a total of 18 books over 3 months.
And then ask us to calculate the numbers of books that he will read in a period of 5 months.
In order to calculate that, we must determine first how many books Kristen read in a month.
# of books that Kristen read in a month = 18 books / 3 months = 6 books / month
So, in order to calculate the number of books that Kristen read in 5 month, we just multiply:
5 months * 6 books/month = 30 books (answer)