答案为4 把后面的4往前提可以约掉5也可以约掉最后只有4
5/4X5/4X1/4X1/4X4/5X4/5X4X4=(5/4X4/5)X(5/4X4/5)X(1/4X4)X(1/4X4)=1X1X1X1=1