We can see that the problem is asking us for the percentage of youth that do not use the Internet at home. Know that there can only be 100% in mathematical models, whihc means that the percent of youth that use the Internet most often somewhere else is:

Now, we are going to need to convert this percentage to a fraction. To convert percentages put the percentage in the numerator in a fraction with 100 as the denominator (percentages are out of 100). In this case, 18% is made into the fraction like so:

The answer is 9/50.