THINK OF A NUMBER.
Tell your neighbour to think of any number he likes, but not to tell you what it is. Tell him then to double it; when he has done that, let him add an even number to it, which you yourself must give him; after doing this he must halve the whole, then from what is left take away the number he first thought of. When he shall arrive so far, if his calculations have all been made correctly, you will be able to give him the exact remainder, which will simply be the half of the even number you told him to add to his own.