一道关于C语言计数控制，条件控制的循环，嵌套循环的题

只做出来前三个实在不知道怎么搞了，求求救救孩子

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

int main()
{
    int x, y, z, m, k, c, i, f=0;
    srand((unsigned)time(NULL));
    for( i=0; i<10; i++ )
    {
        z = rand() % 21;
        x = rand() % z+1;
        y = z - x;
        m = rand() % 2;
        if (m==0)
        {
            printf("%d + %d = ", x, y);
            k = z;
        }
        else
        {
            printf("%d - %d = ", z, x);
            k = y;
        }
        scanf("%d", &c);
        if(c==k)
        {
            f += 10;
            printf("回答正确\n");
        }
        else
        {
            printf("回答错误\n");
        }
    }
    printf("得分：%d",f);
    return 0;
}