210 college students were asked in a survey if they

So this is a Venn Diagram Question.

But first, I should say that this question is poorly worded. And even if the wording were a little better, you would never see a question like this on the actual GMAT.

Why? Because of politics!

OK, you're probably wondering what politics has to do with the GMAT.

Well, just think about it. By throwing a question like this, the GMAC guys presented an example of people slightly favoring Macs over PCs--and they did this in front of hundreds of thousands of GMAT test takers. That's good marketing for Macs--but not so good for PCs.

Do you think the PC guys would be happy about this? Can you imagine the letters of lawsuits or cease and desist letters that GMAC would receive if they made a question like this go live?

So you'd never see this specific type of example on the GMAT exam. It's fine to use sources outside of the OG and GMATPrep--but just keep in mind you probably won't see them on the actual exam.

Additionally, as a few of you have noted, the word "only" needs to be included where appropriate to make the distinction between the categories clearer.

Having said all of the above, the mental diagram you should be picturing inside of your head looks like the above picture.
The overlapped area in the middle is the 20 mentioned in the question.
The 60 presents ONLY Macs.
And the question is asking you how many are on the ONLY PC side.
You are given 210 students surveyed--of them 90 don't really have an opinion.

60 + 20 + 90 + x = 210
170 + x = 210
x=40

Make sure this diagram makes sense so you can apply it to future questions. Hope that helps!