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The output of your command "$ du -sk *.txt" is:

24	How-to-Zone.txt
4	c7.txt
0	del.txt
4	f.txt
4	file1.txt
4	file1and2.txt
4	file2.txt
4	file3.txt
116	man_commands.txt
116	man_fpdf.txt
4	p.txt
4	practiceme.txt
4	pwd.txt
12	sar-info.txt
4	solcmd-ad.txt
4	sortme.txt
4	t.txt
72	usr-include.txt
0	w1.txt