It's a headline technique. As well as including a solid number (always a good headline "trick"), it has the psychological effect of making people think.. "ooh, I gotta read this, because if he/she can do X in Y hours/days, maybe I could too!"
I'm quite into studying headlines as a hobby (yeah, I know) and http://delicious.com/popular is a great source of headlines that clicked with a large number of people. Today the number of "X ways to do Y" or "Z things about A" headlines is reasonably low, but I've seen it go over 50% of the links there many a time.. (HN's editors seem to have a policy of editing out the numbers at the start of headlines, FWIW.. but they don't tend to do with the headlines you're referring to.)
I'm quite into studying headlines as a hobby (yeah, I know) and http://delicious.com/popular is a great source of headlines that clicked with a large number of people. Today the number of "X ways to do Y" or "Z things about A" headlines is reasonably low, but I've seen it go over 50% of the links there many a time.. (HN's editors seem to have a policy of editing out the numbers at the start of headlines, FWIW.. but they don't tend to do with the headlines you're referring to.)