Answer: The required number of books that Ruben had before purchases is 50.
Step-by-step explanation: Given that Ruben bought 6 new books for his collection that increased his collection by 12%.
We are to find the number of books that Ruben had before purchases.
Let x denotes the number of books that Ruben had before purchases.
Then, according to the given information, we have
[tex]x+6=x+12\%\times x\\\\\\\Rightarrow 6=\dfrac{12}{100}x\\\\\\\Rightarrow 6=\dfrac{3x}{25}\\\\\\\Rightarrow 3x=150\\\\\Rightarrow x=\dfrac{150}{3}\\\\\Rightarrow x=50.[/tex]
Thus, the required number of books that Ruben had before purchases is 50.