What is the answer to this promblem?

Question

I need to figure out how to solve f(f(x)), while letting f(x)=4x^-1. My book said it equals 1, but two other sources gave different answers. Help me, please!!

Answer 1

Given:  f(x) = 4x^-1 = 4/x

 

Get f(f(x)) by substituting f(x) for x, and simplify.

 

f(f(x)) = 4 / f(x) = 4 / (4/x) = x

 

What other anwsers do you have?