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scanf("%d",x)改成scanf("%d",&x)
还有其他错误，代码修改如下：

#include <stdio.h>
int main()
{
    double sum = 0,data,n;
    int x,i;
    printf("the number of students:");
    scanf("%d",&x); 
    printf("the scores:");
    if (x>0)
    {
        for(i=0;i<x;i++)
        {
            scanf("%lf",&data);
            sum += data;
            n++;
        }
        n = sum /x;
        printf("average=%.2lf",n);
    }else
        printf("average=0");
    return 0;
}

#include<stdio.h>
int main(void)
{ 
    float sum=0.00,data,n;
    int x,i;
    printf("the number of students:");
    scanf("%d",&x);
    if(x>0)
    {
        printf("\nthe scores:");
        for(i = 0;i<x; i++)
        {
            scanf("%f",&data);
            sum += data;
        }
        printf("\naverage=%.2lf",sum/(1.0*x));
    }
    else
        printf("\naverage=0");
  
    return 0;
}

可以把“the number of student”和“the scores”分别在两次输入（人数输入和成绩输入）之前打出来.
望采纳，多谢！

#include <stdio.h>
int main()
{
    int num,sum = 0,i,score;
    printf("the number of student: ");//提前打印小标题
    scanf("%d",&num);
    if(num<=0)//如果人数小于等于0
    {
        printf("average=0.00");
        return 0;
    }
    printf("the scores: ");//提前打印小标题
    i=num;//保存num的值
    
    for(;i>0;i--)//输入所有成绩
    {
        scanf("%d",&score);
        sum += score;
    }
    printf("average=%0.2f",sum/num);//直接输出平均成绩
    return 0;
}