problem with writing in a file!

[spx2]

this is a code wich is supposed to take all numbers with 2 decimals in Balun.txt and is supposed to divde them by 10000 and then modify them so that they have only 2 decimals and write them back.
can anyone please help me with this.
i'm trying for 2 hours and i can't get it right.

#include <conio.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(){
    FILE *f=fopen("Balun.txt","r+");
    char ch[4000];
    ch[0]=0;
    while(!feof(f))
    {
    fscanf(f,"%s",&ch);
    if(strstr(ch,".00")!=NULL)
    {
     int lungime=strlen(ch);
     int lungimenou;
     double cifra=atof(ch);
      cifra/=10000;
      
      sprintf(ch,"%f",cifra);                          
      ch[strlen(ch)-4]=NULL;
      lungimenou=strlen(ch);
      for(int i=lungimenou;i<=lungime;i++)
                                ch[i]=' ';
      
      fseek(f,SEEK_CUR,-lungime);
      ch[lungime]=NULL;
      fwrite(ch,lungime-1,1,f);
      
if (ferror (f))
      printf ("error!!!\n");

      printf("%s\n",ch);

//getch();
    }
    
    else printf("string\n");
    };
    system("pause");
    fclose(f);
    return 0;}

[con]

could you comment your code please, it makes reading it alot easier.