Half Crazy

The teacher noticed that Al had been daydreaming for a long time. She decided to get his attention.

"Al," she said, "If the world is 25,000 miles around and eggs are sixty cents a dozen, how old am I?

"Thirty-four," Al 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," Al said. "My big sister is seventeen and she's only half-crazy."