Count the # of half tiles, quarters and three-quarters.
Then you gotta guess, because some people thought you could use "leftover" quarters to cover the halves, but I myself did not.
This was a cute problem. Too bad the SRM was canclled, as it would've been the first medium I got since August.
Basically, it's fractions and recursion.
I actually didn't read the whole problem; while I was reading it at about 10:08 (7 minutes before coding was to end), the "This SRM will not be rated" message popped up.
I think it would've been DP, being careful with the fractional probabilities.
Hmm, fractions again.