Lately I have found different versions of this puzzle, I was wondering if it has a name or belongs to a specific kind, so that I can understand it better. Here it is:
Peter got \$10 from his father, \$10 from his mother and went to buy some groceries. He spent only \$17.
Once he is back home he gives back \$1 to his father, \$1 to his mother and he keeps \$1 for himself.
In the end his mother spent \$9, his father spent \$9 and that is \$18. Plus the dollar he kept, \$19. Where has that last dollar gone?