MATH PUZZLE: Can you replace...

The pretty teacher was concerned with one of her eleven-year-old students. Taking him aside after class one day, she asked, "Little Johnny, why has your school work been so poor lately?"
"I'm in love," the boy replied.
Holding back an urge to smile, she asked, "With whom?"
"With YOU!" he said.
"But Johnny," she said gently, "don't you see how silly that is? It's true that I would like a husband of my own someday. But I don't want a child."
"Oh, don't worry," the boy said reassuringly, "I'll use a rubber!"