The most upvoted solution, which has 13 upvotes, is nice, short, and intuitive, but not rigorous. The best rigorous solution has 3 upvotes while mine, which is more complete has 1.
Not really whining, but something seems wrong with the upvote system.
EDIT: He said: 2^1 ends in 2 2^2 ends in 4 2^3 ends in 8 2^4 ends in 6 2^5 ends in 2 2^4k ends in 6.
Why does 2^4k end in 6? It is not explained at all.