给出一百分制成绩，要求输出成绩等级‘A’、‘B’、‘C’、‘D’、‘E’。 90分以上为A 80-89分为B 70-79分为C 60-69分为D 60分以下为E

#include
int main()
{
    float m;
    scanf("%d",&m);
    if(m>100||m<0)
        printf("input is wrong!");
    else
        if(m>=90)
            printf("A\n");
        else
            if(m>=80)
                printf("B\n");
            else
                if(m>=70)
                    printf("C\n");
                else
                    if(m>=60)
                        printf("D\n");
                    else
                        printf("E\n");
                    return 0;
}

你的代码，很多都是错的，第一行就没有预处理命令；
还有m浮点数，而你却写成%d;
还有你自己看一下，吧！！
#include            
int main()
{
    float m;
    scanf("%d",&m);  //此处
    if(m>100||m<0)     //此处
        printf("input is wrong!");
    else
        if(m>=90)       //此处
            printf("A\n");
        else
            if(m>=80)
                printf("B\n");
            else
                if(m>=70)
                    printf("C\n");
                else
                    if(m>=60)
                        printf("D\n");
                    else
                        printf("E\n");
                    return 0;
}

建议用 switch() ...  case....break;  语句