using $\displaystyle y'=k(M-y)y$

I know how to solve this problem, my issue is getting the k.

M=1000 (total population)

at time t=0 there are 80 cases and 4 hours later half the total population (500) are infected.

I need to find the time when 90% of the population is infected.

I need to find my k to solve it though. initially I though $\displaystyle \frac{500-80}{4-0}= 105$ but that's incorrect