Interesting. There must be something unique about the market for apps in Norway as I managed to hit a similar milestone [1] (outselling Angry Birds in Norway for one day) in 2011.

[1] http://trimensional.com/blog/2011/05/23/trimensional-is-1-in...

There is also this one from Forbes. It claims it only takes 42 minutes, but that depends on how you define "learning algebra", i guess. Still a great game that both my kids loved.