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!
