大家看一下这个机经？

1. My first question: A does a job: 300ton/hour, B: 300ton/hour, C:200/2hours. How many hours will take the three working simultaneously to move 300ton sth.?
  Answer: 3/4

答案错了吧……

难道不是4/7？

两个人一起干1/2就干完了，怎么可能是3/4呢？