What is more useful when it ...

The teacher noticed that Johnny had been day dreaming for a long time. She decided to get his attention."Johnny," she said, "If the world is 25,000 miles around and eggs are sixty cents a dozen, how old am I? "Thirty-four," Johnny answered unhesitatingly.
The teacher replied "Well, that's not far from my actual age. Tell me...how did you guess?"
Oh, there's nothing to it," Johnny said. "My big sister is seventeen and she's only half-crazy."