How can I compare a value of time with a time slot?

Question

I am reading from a file (fp) fourplets, that represent packets and if the generation time of the packet is less than the time running, then I am inserting it in to my list. The time increases by 20.0 every time slot. But I want to check all the packets from the file for the time between 0 and 20 and then between 20 and 40 etc. For example if a packet has generation time 13.9 and the time slot is 0-20, that means the packet will be inserted in the list, but another packet with generation time 24.5 will not be inserted in the list. I wrote the code below but it does not work like that.

  double time=0.0;  
  while( fgets(buf,sizeof(buf),fp) != NULL ){
    fscanf(fp, "%d %d %d %lf",&rollnumber, &src, &dest, &gentime);
    printf("time: %.1f\n",time);
    while (gentime<time && insertedpackets<=10){ 
            insert(rollnumber,src,dest,gentime);
            insertedpackets++;
            display();
        }time=time+20.0;