MATH PUZZLE: Can you replace...

A little old lady is walking down the street dragging two large plastic garbage bags behind her. One of the bags rips, and every once in a while a $20 bill falls out onto the sidewalk.
Noticing this, a policeman stops her, and says, "Ma'am, There are $20 bills falling out of your bag."
"'Oh, really? Darn!" says the little old lady. "I'd better go back, and see if I can find them. Thanks for telling me.."
"Well, now, not so fast," says the cop. "How did you get all that money?' You didn't steal it, did you?"
"Oh, no", says the little old lady. "You see, my back yard is right next to the football stadium parking lot. On game days, a lot of fans come and pee through the fence into my flower garden. So, I stand behind the fence with my hedge clippers. Each time some guy sticks his thing through the fence, I say, '$20 or off it comes."
"Well, that seems only fair." laughs the cop. "OK? Good Luck! Oh, by the way, what's in the other bag?''
"Well, you know", says the little old lady, "not everybody pays."