Asurvey showed that 82% of youth most often use the internet at home . what fraction of youth surveyed use the internet most often somewhere else?

When talking about surveys (and most other things) we use 100% as the cap. You can not work harder than 100% efficiency and you can not have more than 100% of people agree with you. You can think of this as a pie.

A whole pie would be 100%
A half a pie 50%
Etc.

You can not have more than a whole pie (without adding another pie or variable in this case)

So when you look at this problem with that analogy in mind, all it is is simple addition and subtraction. 

If you create a survey and 82% of people vote yes, how many, out of 100%, voted yes.

To find this we take 100% (The whole pie or everybody who voted) and - the portion of the pie we ate or everyone who voted yes, 82%, which will give us the answer, everyone who voted no or the pie that is left on the platter.

So your equation would be 100 - 82 =18

18% of people interview used internet someplace other than in the home.