myfun.h - Header file

/*
define macro MY_FUN, this will reduce multiple calling
of the header file
*/

#ifndef MY_FUN
#define MY_FUN

/*function declarations*/
int sumOfNumbers(int x, int y);
float avgOfNumbers(int x, int y);

#endif

myfun.c - Source file

/* 
function name: sumOfNumbers()
this function will return sum of two numbers
*/

int sumOfNumbers(int x, int y)
{
    int sum;
    sum = x + y;
    return sum;
}

/*
function name: sumOfNumbers()
this function will return average of two numbers
*/

float avgOfNumbers(int x, int y)
{
    int sum;
    float avg;

    sum = x + y;
    avg = sum / 2;
    return avg;
}

main.c - Main Source file

#include <stdio.h>
#include "myfun.h"

int main()
{
    int a, b;
    int sum;
    float avg;

    printf("Enter first number: ");
    scanf("%d", &a);
    printf("Enter second number: ");
    scanf("%d", &b);

    /*calling sumOfNumbers()*/
    sum = sumOfNumbers(a, b);

    /*calling avgOfNumbers()*/
    avg = avgOfNumbers(a, b);

    printf("Sum: %d \nAverage: %f\n", sum, avg);

    return 0;
}

Compiling the program

In turboc Compiler: - Just like normal program compilation, press ALT+F9 to compile and CTRL+F9 to run the program.
In gcc/g++ in Linux Compiler: To compile we can write command gcc main.c myfun.c -o main, gcc *.c -o main or gcc -o main *.c .
To Run: write command ./main

Output:

    Enter first number: 10
    Enter second number: 12
    Sum: 22
    Average: 11.000000