Let's create a variable to represent the size of Ruben's book collection prior to his buying the new books.
B = number of books in Ruben's collection before buying new ones
So, we know that when he bought 6 books, this increased his collection by 12%.
12% of his book collection would be represented by 0.12B. Therefore, we know that:
0.12B = 6
Divide both sides by 0.12, to get to B = 50.
However, you are being asked how many books does he have now. B represents the number of books he had before. Therefore you want to add back the 6 books he just bought.
