Scientists have discovered that a model for the spread of Covid-19 is that the rate of spread is jointly proportional to the number of infected people and the number of uninfected people. In an isolated town of 5000 people, 160 people have Covid-19 on the first day (let this be t = 0) and 1200 have Covid-19 on day 8 (so t = 7). How long does it take for 85% of the population to become infected?