b) [5 pts] In a certain city with population of 100 thousand, y(t) thousand people have flu, where t is the number of days since March 1. It has been determined that the rate at which new people get infected is proportional to the number of people who do not yet have the disease. Suppose that 20 thousand have the u on March 1 and that 60 thousand have it 15 days later on March 16. Write and solve an initial-value problem for y(t)