First, you would divide 9/72 by 72/72 and would get .125. So every book weighs .125 kg.
Then, to find how much 80 book weigh, you multiply 80 times .125 and get 10.
Lastly, to find how many books weigh 6 kg you, keep multiplying .125 by a number until you reach 6 kg. In this case the number is 48. So 48 books weigh 6 kg.
