Ionly need the answer to 18 i have no idea how to do this : (

8 months

Step-by-step 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

2 is the final ! : )

8

step-by-step explanation: