Basically you'll use fopen() (twice once for the input file and once for the output file), then fgets() from the input file 20 times, then loop where you fgets() from the input file, fprintf() to the output file, then fclose() both input and output file. If you want, delete the input file and use copyfile() or movefile() to copy the new file with the 230 missing lines to have the same name as the original input file.
It's pretty obvious so I'm sure you already tried it, but what was the error or unexpected results you encountered?
