Ruben has 6 new books for his collection. This increases his collection by 12%
Let x be the number of books Ruben has before 6 new books added
x + 12% of x = x+6
x + 0.12x = x+6
1.12 x = x+6
Subtract x on both sides
0.12x = 6
Divide by 0.12
x = 50
6 new books are added for his collection
So 50 + 6 = 56
He have 56 books now.