Eight hundred people were asked how often they use a library. The relative frequencies recorded were: never 0.15, sometimes 0.35, often (not stated) and always 0.20. How many people answered 'often'?

Relative frequencies over all the categories must add to 1, since between them they account for everybody. So 'often' has relative frequency $1-(0.15+0.35+0.20)=1-0.70=0.30$, and the count is $0.30\times 800=240$ people. The check works: 120 + 280 + 240 + 160 = 800. The other options are the counts for the three categories that WERE given — 120 for 'never', 280 for 'sometimes' and 160 for 'always' — each right for its own row but not for the row asked about. A relative frequency is a fraction of the whole, so it becomes a head count only after multiplying by the size of the survey; quoting 0.30 by itself would answer a different question.