Oh! That's simple. You no need to use the #define, #ifndef & #endif set of directives. Instead of that you should just use #pragma once pre-processor directive. And it will take care of every thing. Below is sample by making small change in the extendedmath.h

#pragma once

#include "SimpleMath.h"

int Add_three_numbers(int, int, int);

int Add_three_numbers(int a, int b, int c)

{

return ( Add_Numbers(a, b) * c );

}