The math teacher notices that …

The math teacher notices that Johnny had been daydreaming for most of the class. To get his attention she says, “Johnny, if the world is 25,000 miles around, and a dozen eggs are 90 cents, how old am I?
“Thirty-four,” Johnny answered without hesitation.
The teacher who is all amazed, replies, “Well, that’s correct.”Tell me, uh. . . how did you figure that out?”
Oh, there’s nothing to it,” Johnny said.”My big sister is seventeen and she’s only half as crazy as you.”