On each corner of a square is a quarter. Your task is to have all four quarter heads-up or tails-up at the end of a turn.

You are blindfolded at the start, and you do not know which are heads-up and which are tails-up. Each turn, you may flip however many of them you want and then ask if you are done (and no, you cannot tell, by touch, whether it is heads- or tails-up). The square is then rotated a random, undisclosed number of quarter spins (multiple of 90 degrees), and you may take another turn.

Minimize the maximum number of turns required to be assured you will complete the task.

i guess it would still be 8 turns since the 7 solution doesn't work if the coins all start out solved which could be the case and you didn't know when you started flipping.

the first move should always be flip 0 (or all 4) coins

then the 7 solution is good

Posted by tanx on 2005-11-07 18:45:05