#include<stdio.h>
int main()
{
    int x,n,y;
    float sum=0.0;
    printf("请选择: 1.日用品 2.文具 3.食品\n");
    scanf("%d",&x);
    switch(x)
         {
         case 1:printf("请选择:1.牙刷(3.5元/支) 2.牙膏(6.2元/支)\n");
                printf("       3.肥皂(2元/块)   4.毛巾(8.6元/条)\n");
                scanf("%d",&y);
                printf("数量?");
                scanf("%d",&n);
                switch(y)
                {
                 case 1: sum=3.5*n; break;
                 case 2: sum=6.2*n; break;
                 case 3: sum=2*n; break;
                 case 4: sum=8.6*n; break;
                }
                break;
         case 2:printf("请选择:1.笔(3元/支) 2.笔记本(1.2元/个)\n");
                printf("       3.文件夹(12元/个) 4.文具盒(8.6元/个)\n");
                scanf("%d",&n);
                switch(y)
                {
                 case 1: sum=3*n; break;
                 case 2: sum=1.2*n; break;
                 case 3: sum=12*n; break;
                 case 4: sum=8.6*n; break;
                }
                break;
         case 3:printf("请选择: 1.白糖(3.6元/包) 2.盐(1元/包)\n");
                printf("        3.饼(2元/个) 4.(3.6元/条)\n");
                scanf("%d",&n);
                printf("数量?"）；
                switch(y)
                {
                 case 1: sum=3.6*n; break;
                 case 2: sum=1*n; break;
                 case 3: sum=2*n; break;
                 case 4: sum=3.6*n; break;
                }
                break;
}
printf("总计:%.2f元\n",sum);
return 0;
}

仔细检查代码便能发现问题，这也许是题主写代码比较乱所导致的问题，已改正

#include<stdio.h>
int main()
{
    int x,n,y;
    float sum=0.0;
    printf("请选择: 1.日用品 2.文具 3.食品\n");
    scanf("%d",&x);
    switch(x)
         {
         case 1:printf("请选择:1.牙刷(3.5元/支) 2.牙膏(6.2元/支)\n");
                printf("       3.肥皂(2元/块)   4.毛巾(8.6元/条)\n");
                scanf("%d",&y);
                printf("数量?");
                scanf("%d",&n);
                switch(y)
                {
                 case 1: sum=3.5*n; break;
                 case 2: sum=6.2*n; break;
                 case 3: sum=2*n; break;
                 case 4: sum=8.6*n; break;
                }
                break;
         case 2:printf("请选择:1.笔(3元/支) 2.笔记本(1.2元/个)\n");
                printf("       3.文件夹(12元/个) 4.文具盒(8.6元/个)\n");
                scanf("%d",&y);////////////////////////原本落了几句
                printf("数量?");///////--------第一个没错，既然结构相同，建议你用复制粘贴代码
                scanf("%d",&n);////////////////////////
                switch(y)
                {
                 case 1: sum=3*n; break;
                 case 2: sum=1.2*n; break;
                 case 3: sum=12*n; break;
                 case 4: sum=8.6*n; break;
                }
                break;
         case 3:printf("请选择: 1.白糖(3.6元/包) 2.盐(1元/包)\n");
                printf("        3.饼(2元/个) 4.(3.6元/条)\n");
                 scanf("%d",&y);//////////////////////
                printf("数量?");//////////////////////
                scanf("%d",&n);//////////////////////
                switch(y)
                {
                 case 1: sum=3.6*n; break;
                 case 2: sum=1*n; break;
                 case 3: sum=2*n; break;
                 case 4: sum=3.6*n; break;
                }
                break;
}
printf("总计:%.2f元\n",sum);
return 0;
}

感谢！