> If the integers from 1 to 999,999,999 are written as words, sorted alphabetically, and concatenated, what is the 51 billionth letter?

In a series of posts, mathematician Dylan Thurston and computer scientist / linguist Chung-chieh Shan solve this problem step by step, introducing concepts such as monoids and differentiation along the way, use the programming language Haskell.

Epilogue: Discussion with a representative from ITA Software, the creators of the problem: http://conway.rutgers.edu/~ccshan/wiki/blog/posts/WordNumber...

I specifically remember this problem from ITA's advertisements on the MBTA.