MATH PUZZLE: Can you replace...

I was sitting at a bar one time, when I noticed that, next to me, an old drunk was hassling one of the biggest, toughest guys I'd ever seen.

The old guy was clearly blasted, and kept getting in the tough guy's face, say, "I fucked your mother."

Despite being huge and jacked, the tough guy just kept shrugging it off. The old guy laughed in the tough guy's face, saying it again. "Hey, I fucked your mother."

Then, the old man even poked him, and repeated himself, "No seriously, I fucked your mother."

At this point, finally, the tough guy had had enough. He grabbed the old man by his jacket and began to pull him out of the bar, yelling,

"That's it. We're going home, Dad. You're drunk."