程序代码：

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

void tra_arr(int * a, int len);
float get_average(int a[], int length);
void get_arrb(int a[], int length, int t);
int main( void )
{
    int a[20] = {0};
    int i;
    
    float average = 0.0; 
    printf("please you input the 20 number:\n");
    for( i = 0; i < 20; i++)//输入元素 
    {
        scanf("%d", &a[i]);
    }
    
    printf("The a arr:\n");
    tra_arr(a, 20);
    printf("\n");
        
    average = get_average(a, 20);
    printf("The average is %f\n", average);
    
    
    get_arrb(a, 20, average);
    
    system("pause");
    return 0;
}
void tra_arr(int a[], int length)//遍历数组 
{
    int i;
    for( i = 0; i < length; i++ )
    {
        printf("%d\t", a[i]);
    }
}
float get_average(int a[], int length)
{
    float average = 0.0;
    int i;
    int sum = 0;
    for ( i = 0; i < length; i++ )
    {
        sum += a[i];
    }
    average = 1.0 * sum / length;
    return average;
} 
void get_arrb(int a[], int length, int t)
{
    int i;
    int n = 0;
    int temp;
    for (i = 0; i < length; i++ )
    {
        if (t > a[i])
        {
            n++;
            
            printf("%d\t", a[i]);//目前卡在这里，我只能找出小于平均值的数，实现不了排序 
        }
        
    }
    printf("%d\n", n);
    
    
}