Mathematician Shen Ye wrote an equation last year that predicted Android's app store growth to within a day of Google Play hitting 25 billion downloads.
Calculated about a year ago, Ye's equation — (8.6*x)^3.97728 — predicted that the Android Market, as it was then named, would reach 11.9 billion downloads by the end of December 2011. Following Ye's curve, which can be seen here in a Google Drive spreadsheet, the store would reach 25,323,591,312 downloads by the beginning of October this year.
If the trend continues in the same vein, then Ye's equation predicts that Google Play will double its number of downloads (50 billion, total) by July of next year.
Ye, himself, said that the maths is nothing amazingly special, in and of itself, but the lucky accuracy is mind-blowing.
We'll just be bookmarking that spreadsheet, though, for future reference. Any chance of an Apple version?