Exponential growth

Given: There are 7.3 billion, that is, 7.3*10^9
people on Earth. If a flu virus starts
with one person and within 1 day four more
people are infected, then each of those four
people in 1 day infects 4 people each,
and so on, how many days before everyone
on earth is infected?

Solution:

This is an exponential growth problem where the population quadruples every day. Our model equation is:


with
, 
which leads to...





t is about...16.4 days