There are 4000 textbooks in a classroom. 45% of them are math. 40% of the math textbooks are junior. 60% of the junior textbooks are new. What percent of the textbooks are used?

Question

Please answer the following question 

There are 4000 textbooks in a classroom. 45% of them are math. 40% of the math textbooks are junior. 60% of the junior textbooks are new. What percent of the textbooks are used?

Answer 1

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%.