Robby goes to the supermarket to buy a bag of chips. He brings $20 to the store and pays $3.79 for the chips. On the way home, he finds $1.35 in coins on the sidewalk. How much money does Robby have when he gets home?
you must subtract to obtain the quantity that Robby has when he has  bough  the chips ,20- 3.79 =  $ 16,21 but when he return to home he finds $ 1.35, then he has $ 1.35 + $ 16,21 =  $17,56
