During a certain epidemic, the number of people that are infected at any time increases at a rate proportional to the
number of people that are infected at that time. If 1,000 people
are infected when the epidemic is first discovered, and 1,200
are infected 7 days later, how many people are infected 12 days
after the epidemic is first discovered?

9514 1404 393

  1367 people

Step-by-step explanation:

The wording "a rate proportional to the number" indicates the function is exponential. The growth factor is 1200/1000 = 1.2 in 7 days, so the exponential function can be written ...

  i(t) = 1000(1.2^(t/7))

After 12 days, the number infected is modeled as ...

  i(12) = 1000(1.2^(12/7)) ≈ 1367

About 1367 people are infected 12 days after discovery.