是输入两位数的成绩，然后显示出成绩等级，90-100是A，80-89B……
运行时总是不显示等级



#include<stdio.h>
int main()
{
    int a,b;
    printf("Enter numerical grade:");
    scanf("%d",a);
    b=a*10;
    switch (b){
        case 10:
        case  9: printf("Letter grade:A");
                     break;   
        case  8: printf("Letter grade:B");
                     break;
          case  7: printf("Letter grade:C");
                     break;
        case  6: printf("Letter grade:D");
                     break;
          case  5: case 4: case 3: case 2: case 1: case 0:
                   printf("Letter grade:F");
                     break;
        default: printf("ERROR");
                   break;
    }
     system("pause");
}