C语言程序设计教程（第三版）课后习题5.6 （C语言代码）

哈哈我是笨b，没想到用除法

int a;
    scanf("%d",&a);
    switch(a/10)
    {
        case 10:
        case 9:
        putchar('A');
        break;
        case 8:
        putchar('B');
        break;
        case 7:
        putchar('C');
        break;
        case 6:
        putchar('D');
        break;
        default：
        putchar('E');
        
    }

#include<stdio.h>

int main(){
	int a;
	scanf("%d",&a);
	switch(a/10){
		case 10:
		case  9:printf("A\n");break;
		case  8:printf("B\n");break;
		case  7:printf("C\n");break;
		case  6:printf("D\n");break;
		default :printf("E\n");break;
	}
	return 0;
}

#include<stdio.h>
int main()
{
  
	char grade;
	scanf("%c",&grade);
	printf("Your score:");
	switch(grade)
	{
	case'A':printf("90~100\n");break;
	case'B':printf("80~89\n");break;
	case'C':printf("70~79\n");break;
	case'D':printf("60~69\n");break;
    case'E':printf("<60\n");break;
    defult:printf("enter date error!\n");
	}
	return 0;
	}
大神们，哪错了？？抱拳了

#include<stdio.h>
int main()
{
 int grade;
 scanf("%d",&grade);
 printf("请输入分数：");
 if(grade<100&&grade>=90)
 printf("A");
 else if(grade<=89&&grade>=80)
  printf("B");
  else if(grade<=79&&grade>=70)
   printf("C");
   else if(grade<69&&grade>=60)
    printf("D");
    else if(grade<=59&&grade>=0)
     printf("E");
  return 0;
}
大佬，为什么答案错误啊

int main()
{
    int grade;
    scanf("%d", &grade);
    switch (grade / 10)
    {
    case 10:
    case 9:printf("A\n"); break;
    case 8:printf("B\n"); break;
    case 7:printf("C\n"); break;
    case 6:printf("D\n"); break;
    case 5:
    case 4:
    case 3:
    case 2:
    case 1:printf("E\n"); break;
    default:printf("输入错误\n"); break;
    }
    return 0;
}

#include<stdio.h>
int main()
{int grade,i;
printf("请输入你的成绩:");
scanf("%d",&grade);
if(grade<0&&grade>100)
printf("成绩无效");
else
{i=grade/10;
    switch(i)
      case 10:
      case 9:printf("A");break;
      case 8:printf("B");break;
      case 7:printf("C");break;
      case 6:printf("D");break;
      default:printf("E");
}
return 0;
}
大神们这个为什么不行啊

#include<stdio.h>
int main()
{
	int grade;
	scanf("%d", &grade);
	if (grade >= 90 && grade <= 100)
	{
		printf("A");
	}
	else if (grade < 90 && grade >= 80)
	{
		printf("B")
	}
	else if (grade < 80 && grade >= 70)
	{
		printf("C")
	}
	else if (grade < 70 && grade >= 60)
	{
		printf("D")
	}
	else
	{
		printf("E")
	}
	return 0;
}
大佬们，这个为什么会有编译错误？？？