How to create this kind of header files in c?

Question

How can i use header code to i.e. make the below code work correctly?(define a function(GET) to scan n and change n mod 2 is 0 and n mod 5 is 0 to n%2==0 && n%5==0)

int main() {
int n;
GET(n);
if(n mod 2 is 0 and n mod 5 is 0)
printf("OK");
else
printf("Not OK");
}

Answer 1

For custom header files.
Write a function inside a separate file and store it with .h extension. Now open a new file and use the header as #include "filename.h" and use the function to perform some operations in the main method of the new file.

Answer 2

what should i do about this code i wrote ( i want to use macros but i dont want to create another file)