I have a problem and a solution, but the problem is nice, so lemme tell you about it first..
The problem is as follows:

We have got 31 tracks and 10 trains, Each train is so big, that between two there needs to be at least one free track.
Now, how many valid combinations do exist to place the 10 trains on the 31 tracks?

Read on if you think that you have got the solution and want to compare it with mine (no guarantee that I've got it right though..)
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