Running at their respective constant rates, machine X takes 2 days longer to produce w widgets than machine Y. At these rates, if the two machines together produce 5/4 w widgets in 3 days, how many days would it take machine X alone to produce 2w widgets? 4 6 8 10 12 之前那个男老师录音列的方程解起来就要好久。。。。麻烦老师再讲一下！