Finding margin of error
"The following quotation is from a recent survey of Internet users. The sample size for the survey was 1371. Since 18% of those surveyed said they download music, the sample size for this subsample is 247. Among current music downloaders, 38% say they are downloading less because of the RIAA suits...bout a third of current music downloaders say they use peer-to-peer networks....24% of them say they swap files using email and instant messaging; 20% download files from music-related Web sites like those run by music magazines or musician homepages. And while online music services like iTunes are far from trumping the popularity of file-sharing networks, 17% of current music downloaders say they are using these paid services. Overall, 7% of Internet users say they have bought music at these new services at one time or another, including 3% who currently use paid services.
For each percent quoted, give the margin of error. You should express these in percents, as given in the quote."
For other problems where I calculated margin of error, I was given a confidence level or a value for z. If that isn't given, how do you know what to use?