So here is how you find the answer of the given problem above.
Given that y = number of CD that Michael has and x is John's.
So x = 15 + y
The total of their CDs is 55.
55 = 15 + y
55 - 15 = y
So y = 30
x = 15 + 30
x = 45
Therefore, John's number of music CDs is 45 and Michael has 30. Hope this answer helps. Thanks for posting your question.