Write a story that could describe the expression:

75x - 20

If I make 75 dollars in a day and then use 20 dollars for gas then how much money will I have left over

If John wants to buy 75 packs of gum but doesn't know how much pieces of gum are in each pack as well as that he has to give 20 pieces to his friend Dan, how many pieces of gum does he have left?

Since he adds x pieces of gum for each pack, his total gum is 75x. Next, since he gives 20 pieces to his friend, we have to subtract 20 from that