int k=(int)rand()%8;
        switch(k)
        {
            case 0:
                printf("%d/%d*%d/%d=",i,j,x,y);
                scanf("%d%c%d",&a[0],&l,&a[1]);
                f=i*x;
                
                g=j*y;
                if(a[0]/a[1]==f/g)
                {
                    printf("答对\n");
                    c=c+1;}
                else printf("答错\n");
                break;
            case 1:
                 printf("%d/%d÷%d/%d=",i,j,x,y);
             
                scanf("%d%c%d",&a[0],&l,&a[1]);
                f=i*y;
                
                g=j*x;
                if(a[0]/a[1]==f/g)
                {
                    printf("答对\n");
                    c=c+1;}
                else printf("答错\n");
                break;
            case 2:
                 printf("%d/%d+%d/%d=",i,j,x,y);
              
                scanf("%d%c%d",&a[0],&l,&a[1]);
                f=i*y+j*x;
                
                g=j*y;
                if(a[0]/a[1]==f/g)
                {
                    printf("答对\n");
                     c=c+1;
                }
                else printf("答错\n");
                break;
            case 3:
                printf("%d/%d-%d/%d=",i,j,x,y);
               
                scanf("%d%c%d",&a[0],&l,&a[1]);
                    f=i*y-j*x;
                
                g=j*y;
                if(a[0]/a[1]==f/g)
                {
                    printf("答对\n");
                     c=c+1;
                }
                else printf("答错\n");
                break;
            case 4:
                printf("%d+%d=",s,o);
              
                scanf("%d",&w);
                f=s+o;
                if(w==f)
                {
                    printf("答对\n");
                     c=c+1;
                }
                else printf("答错\n");
                break;
            case 5:
                printf("%d-%d=",s,o);
              
                scanf("%d",&w);
                f=s-o;
                if(w==f)
                {
                    printf("答对\n");
                     c=c+1;
                }
                else printf("答错\n");
                break;
            case 6:
                printf("%d*%d=",s,o);
              
                scanf("%d",&w);
                f=s*o;
                if(w==f)
                {
                    printf("答对\n");
                     c=c+1;
                }
                else printf("答错\n");
                break;
            case 7:
                printf("%d÷%d=",s,o);
              
                scanf("%d",&w);
                f=s/o;
                if(w==f)
                {
                    printf("答对\n");
                     c=c+1;
                }
                else printf("答错\n");
                break;