An epidemic is reported to have broken out in Florida

Last year, there were n instances of the disease.
This year, the instances have increased by 85% i.e. this year, there are (n + 85% of n) instances of the disease.
The number of newly detected instances = 85% of n

Now the question is: What is the lowest number of newly detected instances possible?
This means we need to find the minimum value of 85% of n.

What are the constraints because of which there must be a minimum value of 85% of n?
The number of cases detected last year and the number of newly detected cases must be an integer. You cannot detect half a case, right?

Can (85/100 * n) be 1? No because in that case, n will not be an integer.
Can (85/100 * n) be 5? No because in that case, n will not be an integer.

Notice that 85 = 5*17.
So 85/100 = 17/20

(17/20 * n) = 17
n = 20 (an integer)

We will not get n as integer in any of the other options.