1 sandwich cut into 8 pieces then eats 1 pieces,
then had 7/8 lefts means that 7 pieces left, if he put each mean put 1 pieces in a plate, than he need 7 plates.
When he ate 1 piece, it means that he ate 1/8 or 1 out of 8 pieces.
he put 1/8 part of the sandwich on each plate.