MATH PUZZLE: Can you replace...

After the baby was born, the panicked Japanese father went to see the obstetrician.

"Doctor," he said, "I don't mind telling you, but I'm a little upset because my daughter has red hair. She can't possibly be mine."

"Nonsense," the doctor said. "Even though you and your wife both have black hair, one of your ancestors may have contributed red hair to the gene pool."

"It isn't possible," the man insisted. "We're pure Asian."

"Well," said the doctor, "let me ask you this. How often do you have sex?"

The man seemed ashamed. "I've been working very hard for the past year. We only made love once or twice a month."

"There you have it!" the doctor said confidently. "It's just rust."