Answer:
The differential equation for the model is

The model for P is

At half day of the 4th day (t=4.488), the population infected reaches 90,000.
Step-by-step explanation:
We can write the rate of spread of the virus as:

We know that P(0)=100 and P(3)=100+200=300.
We have to calculate t so that P(t)=0.9*100,000=90,000.
Solving the diferential equation


Then the model for the population infected at time t is:

Now, we can calculate t for P(t)=90,000

At half day of the 4th day (t=4.488), the population infected reaches 90,000.