Calculating SGPA of a Semester

Problem Statement
Calculating SGPA of a semester

#include<stdio.h>
int main()
{
   float sum,sgpa,tempmin,tempmax,mark;
   char b;
   int sub,i;
   tempmin=tempmax=0;
         printf("enter the grades \n");
         printf("enter x for ex \np for pass \na for a\nb for b\nc for c\nd for d\np for pass\n any other for fail\n");
         printf("subject codes and resp. credits are\n");
         printf("1 for maths credit=4\n");
         printf("2 for chem credit=4\n");
         printf("3 for pds credit=4\n");
         printf("4 for mech credit=4\n");
         printf("5 for pds lab credit=2\n");
         printf("6 for engg. drawing credit=3\n");
         printf("7 for chem lab credit=2\n");
         for(i=1;i<11;i++)
          {
            sum=0.0;
             for(sub=1;sub<8;sub++)
             {
                printf("enter the grade in subject %d of student %d\n",sub,i);
                b=getchar();
                getchar();              /* dummy getchar to eat enter */
                switch (b)
                  {
                       case 'x':
                            mark=10;
                            break;
                       case 'a':
                            mark=9;
                            break;
                       case 'b':
                            mark=8;
                            break;
                       case 'c':
                            mark=7;
                            break;
                       case 'd':
                            mark=6;
                            break;
                       case 'p':
                            mark=5;
                            break;
                       default:
                            mark=0;
                            break;
                     }
                          if(sub<=4)
                          {
                            sum=sum+mark*4;
                          }
                          else if(sub==5||sub==7)
                          {
                               sum=sum+mark*2;
                               }
                          else
                          {
                              sum=sum+mark*3;
                          }
                        
           }       
           sgpa=sum/23.0;   
           printf("the sgpa is %f \n",sgpa);
           if(sgpa>tempmax)
            {
               tempmax=sgpa;
            }
            if(sgpa<tempmin)
            {
               tempmin=sgpa;
            }
     }
     printf("the maximum and minimum sgpa is %f ,%f resp.\n",tempmax,tempmin);
return 0;
}