Answer: 8 months
Explanation:
The difference between how many CD's they have currently is 16 CD's.
The difference in the amount of new CD's they collect each month is 2 CD's.
So if we do 16 divided by 2, the answer to this problem is 8 months.
We can check this by doing 8 x 4(Amount of CD's bought by Mark each month) which is 32. We can add this to his current total to get 77 CD's.
We can do 8 x 2( Amount of CD's bought by Corinna each month) which is 16. We can add this to her current total to get 77 CD's