Solution

avg.c

#include <stdio.h>
#include <stdlib.h>

double avg(int[], int);

double avg(int nums[], int count)
{
    int i, sum = 0;

    for (i = 0; i < count; ++i)
    {
        sum += nums[i];
    }

    return (double)sum / count;
}

int main(int argc, const char *argv[])
{
    int i, nums[argc - 1];

    for (i = 1; i < argc; ++i)
        nums[i - 1] = atoi(argv[i]);

    printf("%f\n", avg(nums, argc - 1));
    return 0;
}