At a branch of UT there are approximately 30000 students. A flu epidemic is spreading among these students at a rate proportional to the product of the number of infected students and the number of students who have not been infected. Initially 1000 students were infected, and 10 days later 4000 were infected. What per- centage of the student body was infected after 20 days? (Hint: to simplify the algebra, let N(t) be the number of infected students (in multiples of 1000).) Enter your answer as a decimal with four significant digits