I hesitate to chime in, not only because I'm all too often obviously wrong here and probably especially so since I've only briefly scanned the responses, but also because I'm not sure how to read the OP. "There are 13 separate piles of coins. Each pile has 4 identical coins in it." Does that mean that in the pile with the odd coins, 4 are like all the rest and 9 are off by delta? And if so and you are picking only 1,2,3,or 4 coins, couldnt you pick all normal coins even from the pile containing odd ones? Or does this line, "The 1 pile of counterfeit coins look and feel identical to the genuine coins." mean all the coins in the counterfeit pile are off by delta? Like I said tho, am probably missing something obvious...