Final answer:
The estimated number of virus particles, in billions, after 4 days is 52.71 billion.
Step-by-step explanation:
To find the estimated number of virus particles, we can substitute x = 4 into the function:
y = -0.72*(4 4) + 3.2*(4 3) + 25.9
Calculating this equation, we get:
y ≈ 52.71 billion
Therefore, the estimated number of virus particles, in billions, after 4 days is 52.71 billion.