We can solve this by setting up a couple of equations.
Let's allow x to represent the number of paperbacks Tim owns, and allow y to represent the number of hardcover books he owns.
Using the information in the question, we can write the equations:
1)

2)

Let's rearrange equation 1 so that it is in standard form:

And then let's multiply equation 2 by 4 so that we can cancel out y when we solve the system of equations:


Then we can add the two equations and solve for x:
1)

+ 2)

------------------------------------


So now we now the number of paperback books Tim has is 357. Let's plug this into one of the original equations to solve for the number of hardcover books (y):


And now we know that
Tim owns 90 hardcover books.