very concise (condensed) but logical, although computational complexity may be reduced by keeping track of already taken instead of recalculating them each time from scratch

what

If you understand how to do it, then you know why there are many tags.

He has to repost the example tests somewhere because example tests don't update or reset

I didn't know how to do it - I'm not good in such katas. I think there is too much tags - numbers, algorithms, mathematics - it's not the topic of this kata.

Great math riddle - love such ones! Some 1e6 test cases would make it more tricky :)

Actually it was the fixed test that was broken. Fixed now.

Do your random tests work?
http://screencast.com/t/Vj66X8aPtQ

Yeah, it's rather simple task, but try change your attitude: for absolute beginners it may be good point to start analysis of syntax and make common with language concepts, even with using = as assigning variable.

Correcting simple typos won't teach coding - it would only teach how to correct typos. When you put : instead of = in variable assigment, better make some clear code showing what will those operators do and how they differ. Those aren't common mistakes. I can't imagine someone trying to assign variable with : insted of = nor someone returning cleraly named function argument just next line after assigning another clearly named var.

So, marty-wang, if you have any ideas how improve this Kata let me know!

It's good idea, so I added link to description. In fact it's so obvious (when you know) that I've had problem how describe it, and write tests, without spoiling solution. Thanks!