MATH PUZZLE: Can you replace...

There were two old geezers living in the backwoods. Their names were Rufus and Clarence. They lived on opposite sides of the river, and they hated each other.

Every morning, just after sunrise, Rufus and Clarence would go down to their respective sides of the river and yell at each other. "RUFUS!!" Clarence would shout. "You better thank your lucky stars that I can't swim . . . or I'd swim this river and whup your butt!!"

"CLARENCE!!" Rufus would holler back, "You better thank YOUR lucky stars that I can't swim . . . or I'd swim this river and whup your skinny butt!!!"

This went on every morning. Every day. Twenty years.

One day the Army Corps of Engineers came and built a bridge. Still, every morning every day for another five years the shouting and feuding across the river continued. Finally, Rufus wife has had enough.

"Rufus!" she yells, one day. "I can't take no more!! Every day for 25 years, you've been threatenin' to whup Clarence. Well, there's the bridge...have at it!"

Rufus thought for a moment. Chewed his bottom lip for another moment. "Woman!" he declared, snapping his suspenders into place. "I'm gonna whup Clarence's butt!!!"

He walked out the door, down to the river, along the riverbank, came to the bridge, stepped up onto the bridge, walked about halfway, looked up....TURNED TAIL AND RAN SCREAMING BACK TO THE HOUSE, SLAMMED THE DOOR, BOLTED THE WINDOWS, GRABBED THE SHOTGUN AND DIVED, PANTING AND GASPING, UNDER THE BED!!!!!

"Rufus!" cried the misses. "I thought you was gonna whup Clarence's butt!!!"

"I was, woman, I was!!" he whispered.

"Rufus!" cried the misses. "What in tarnation is the matter?"

"Well," muttered the terror-stricken Rufus, "I went to the bridge...I stepped up on the bridge...walked halfway over the bridge...looked up..."

"And?" she asked, breathless with suspense.

"And," continued Rufus, "I saw a sign that said, "Clearance, 13 feet, 6 inches" He ain't never looked THAT big from the other side of the river!!!!!"