Finding the change in percent?

I know this should be a lot easier than it is, but I was up late and I've gone and confused myself!

Please read the whole description to make sure you are actually answering what I'm asking...

If I want to find the change in percent of a fraction, how do I do this?

In other words:

If 11 out of 28 (11/28) students answer the question right on the pretest and 20 out 28 (20/28) students answered the question right on the post-test, what is the percent change of students who answered the question correctly?

I know that 39% answered the question correctly the first time, and 71% answered correctly the second, so do I just subtract 39 from 71, or do I do (20-11)/11, or (71-39)/71 or...?

Thanks for your help!