A professor is trying to figure out how many custom books to order for his class of 25 students. In the​ past, the number of students buying books has shown the following demand pattern: Number of students who bought a book Percentage of Observations 16 or fewer 0% 17 4% 18 15% 19 17% 20 18% 21 26% 22 10% 23 6% 24 4% Suppose each custom book costs the professor ​$ 12 to​ print, and he sells the books to the students for ​$ 35 each. Excess books are sold for $3 each and have to be shipped to those buying an excess book. The shipping cost is $1 per book. How many books should the professor plan on ordering? Round to nearest whole number.