Start the problem by guessing one of the extremes (0,0 12,0 0,12 12,12). After this you will get a reading, for example, 10. This shows how far away your guess is from the treasure. This allows you to determine all possible locations for the treasure, which I have marked on my diagram with x’s. After this, guess one of the ends of the possible locations for the treasure. If you are lucky, then this will be where it is, but this is unlikely. 

After guessing one of these numbers, (if you guessed 0,0 for the starting location, and received a reading of ten away, you would either guess 10,0 or 0,10), you will receive a reading that tells you the exact location of the treasure. If I receive a reading a reading of 8 away from my 10,0 guess, then I will know that the answer is 6,4, because it is 8 away from 10,0, and 10 away from 0,0.