We'll have four weeks' holiday.中为何不用week's或weeks,而用 weeks'?

We'll have four weeks' holiday.中为何不用week's或weeks,而用 weeks'?
We'll have four weeks' holiday.
中为何不用week's或weeks,而用 weeks'?

We'll have four weeks' holiday.中为何不用week's或weeks,而用 weeks'?
four weeks表示四周,而'是代表’s,因为前边week已经加了s,所以省略了s,你说的那两种都是不正确的