Where has the last dollar gone?

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?

• Those crazy Americans... When will they finally use proper SI units for their money instead of these MathJax markers? – A. P. Feb 7 '18 at 23:35

The truth is Mum and Dad both spent 9.50. $$17 / 2 = 8.5$$ After they were given 1 back. $$8.5 + 1 = 9.5$$ $$9.5 \times 2 = 19$$