composite function

Question

Let the function g: N -> N be defined by g(x) = x for any n belongs N . find a value of  x belongs to N , such that g(g(g(x))) = 1000 explain your solution

Answer 1

g(x)=x

g(g(x))=g(x)=x

g(g(g(x)))=g(x)=x=1000. So, x=1000.