問答題 Some people read tea leaves to predict the future. We prefer hamburgers! The Economist began to calculate Big Mac index in 1986. They use it as a light-hearted guide to whether currencies are at their “correct” exchange rate. It is not intended as a precise predictor of exchange rates, but a tool to make economic theory more digestible. The following is the Big Mac Index in Apr. 25th 2002.   詳細解釋購買力平價說，說明其均衡是如何實現(xiàn)的，為何這是一個長期學說。