C++ :: Where To Put The Include Directive And Using Namespace Std With Multiple Files

I have two files that I want to compile:

main.cpp
Code:
#include <iostream>
using namespace std;
int ReadNumber();
void WriteAnswer();

[Code] .....

The compiler complains:
io.cpp||In function 'int ReadNumber()':|
io.cpp|3|error: 'cin' was not declared in this scope|
io.cpp||In function 'void WriteAnswer()':|
io.cpp|7|error: 'cout' was not declared in this scope|
io.cpp|7|error: 'endl' was not declared in this scope|

In io.cpp file, should I put the two statements ("include <iostream>" and "using namespace std") at the top, outside of the functions?

Or should I put the two statements inside each of the functions?