Ruben bought 6 new books for his collection. this increased his collection by 12%. how many books does he have now?

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.

The answer is 50 + 6 = 56