Feeling kinda nerdy....
Now I do not disagree with the math that NeverhadaPC suggested...if anything I used that method the last time they hit the X-billionth song, because I wanted the macbook pro.
But there were two things that I realized at the last contest:
One, the speed to reach the given download is between a linear and exponential growth speed. Right now, what we are witness is mostly casual buyers and a small % of people trying to push ahead faster to 10 billion.
Two, as we draw closer to 9,999,999,999 the speed increases and a "stop and go" effect starts. You would notice as the deadline comes, the counter will stop and then go and then stop and then go. Which makes buying the 10 billionth song all based on luck really...
Lastly, your chances of getting the 10 billionth song is 1/(all the people trying to buy the one billionth at the right time). The number can be 1/10,000 or it can be high as 1/10,000,000...comparable to winning 2nd prize of most big game lottos.