three and a half years,three and half years 的区别?

three and a half years(三年半)
=three years and a half

而 three and half years是错误的。

three and a half years(三年半)
=three years and a half

We spent --three and a half years-- to build that bridge.

She is --three and half years-- old.

前者是作状语,后者作定语
three and half years怎么会有错呢??
当然了,three years and a half也是对滴~~也是作状语