45% of 4000=0.45*4000=1800 math books.
40% of these=0.40*1800=720 junior math books.
60% of these=0.60*720=432 new junior math books.
Used (not new) junior math books=720-432=288. This is 40% of 720.
As a percentage the used ones are 288/4000*100%=7.2%.
The same result is achieved if we calculate 0.45*0.40*0.40=0.072=7.2%.