Let numbers of books be 'b' and numbers of CDs be 'c'
We can set up two equations:
Equation [1] ⇒
Equation [2] ⇒
We are solving for the number of books and the number of CDs bought
When we have two equations in terms of two different variables;
and 
, that we need to solve, then this becomes a simultaneous equation problem.
First, rearrange Equation [1] to make either or the subject:
Then we substitute into Equation [2]
Now we know the value of which is , substitute this value into we have 
Answer:
Numbers of books = 13
Numbers of CDs = 7
We can set up two equations:
Equation [1] ⇒
Equation [2] ⇒
We are solving for the number of books and the number of CDs bought
When we have two equations in terms of two different variables;
First, rearrange Equation [1] to make either
Then we substitute
Now we know the value of
Answer:
Numbers of books = 13
Numbers of CDs = 7