It´s＋fair+< >sb+to do sth

填for
fair解释为：公平的；公正的

句型：It's + adj. + for/of sb. to do sth.

用for的，则adj.不能用来修饰sb.
eg.It's necessary for you to read books.（不能说a necessary man吧)

用of的，则adj.能修饰sb.
eg.It's foolish of you to play computer games all the time.(可说a foolish man)

of