From the topic, the class size is a multiple of12,6,4. The least common multiple of these three numbers is 12. 1/6=2/ 12 1/4=3/ 12

So, you need to buy 7 popular science books, 2 fairy tales and 3 history books. Only in this way can we ensure that every student in the class has the same possibility to read this book. (Of course, you can also buy multiples of these numbers for the same reason. )