The old car goes a mile up the hill, and then a mile down the hill. It can only go 15 miles an hour up the hill. How fast must the car go down the hill in order to average 30 miles per hour for the entire two-mile trip? Duh, 45 miles per hour. But the explanation in this video went right over my head. Why does it matter how much time the trip takes? Why bring time into it at all? They lost me completely. Can you explain it any simpler than this guy does? -via Digg
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