why A=1?

I need a detail only for this

A rumor spreads through a school with 2000 students. At 8 AM. 100 students have heard the rumor, and by noon, half the school has heard it. Using the logistic model M) = determine when 90% of the students will have heard the rumor. (Use decimal notation. Give your answer to one decimal place.) Let y(t) be the proportion of students that have heard the rumor at a time t hours after 8 AM. Use the logistic model with a capacity of A = 1 (100% of students) and an unknown growth factor of k. 1 Vt) = 1 - v