Adam’s bookshelf has 4 shelves that fit 20 books each, which is 4 * 20=<<4*20=80>>80 books in total to fill the bookshelf
Adam ended the day with a full bookcase of 80 books in the bookcase and 2 books left over, 80 + 2= <<80+2=82>>82 books total.
If Adam started the day with 56 books and ended the day with 82 books then Adam bought 82 - 56= <<82-56=26>>26 books bought on his shopping trip.
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