Display contents of .txt file

[karmo]

I display whole .txt file:

QFile fp("file.txt");
fp.open(IO_ReadOnly);
while (!fp.atEnd() ) {
	fp.readLine(buff, sizeof(buff));
	textEdit->append(buff);
}
fp.close();

QFile fp("file.txt");
fp.open(IO_ReadOnly);
while (!fp.atEnd() ) {
	fp.readLine(buff, sizeof(buff));
	textEdit->append(buff);
}
fp.close();

To copy to clipboard, switch view to plain text mode 

but I would like to display last 20 lines or next to last 20 lines. How can I achive this?