问题如下：
Interest rates in a currency XXX increase and interest rates in currency YYY stay the same. The exchange rate is expressed as XXXYYY. Do forward rates increase or decrease?
选项：
解释：
Currency XXX becomes weaker in the forward market. Exchange rates are expressed as the number of units of YYY that would be exchanged for one unit of XXX. As a result of the increase in interest rates, it takes less units of YYY to buy one unit of XXX in the forward market and so the forward rate decreases.
答案明白，就是As a result of the increase in interest rates, it takes less units of YYY to buy one unit of XXX in the forward market and so the forward rate decreases. 这里forward rate的公式是什么，前面的解释都明白，为什么因为XXX贬值了，YYY在同样的unit里花比雨原来的钱少来购买XXX，就说明forward rate就降低了呢？XXX是base currency吧? 烦请老师解答一下，谢谢！
