"In ten seconds or less write down the exact number of permutations that number of coins produces when flipped..." - F. Pohl. #readingToday 

_If you draw a row of coins, it doesn't matter how long a row, I will in ten seconds or less write down the exact number of permutations (heads-tails-heads, heads-tails-tails, etc.) that number of coins produces when flipped. And just to make it a little tougher on me, I will do it even if you cover up as many coins in the row as you like, from either end, so that I won't be able to tell how many there are in the row.
Impossible, right? Care to try to figure it out? I'll come back to you on this._