I am not sure what the question is here. I can tell you how to find the percent of kids that developed influenza in that class. We can compare it to the 20% of students nationwide. There could many different questions that stem from the piece of information you gave.
To find the percent of kids that developed influenza in that class do 6÷15 which gives you
.4 and written as a percent would be 40%.
Please message me if you have any questions! Hope this helps!
Option C: (x+4) and (x-1) are the factors of the equation
Explanation:
Given that the equation is 
We need to determine the factors of the equation.
Splitting the middle term, we get,

Grouping the terms, we have,

Let us factor out the term (x+4), we have,

Thus, the factors of the equation are (x+4) and (x-1)
Hence, Option C is the correct answer.
Answer:
11/2, 22/4, 33/6, 44/8, 55/10, 66/12, 77/14, 88/16, 99/18, 110/20, 121/22, 132/24, 143/26, 154/28, 165/30, 176/32, 187/34, 198/36, 209/38, 220/40, and so on ...