The cost of thirteen CDs and seven DVDs is $99.80; and the cost of seven DVDs and twelve CDs is $84.11.

want do you need to find?

this problem is not complete

13c+7d=99.8

12c+7d=84.11
Multiply the 2nd equation by -1
13c+7d=99.8
-12c-7d=-84.11
c=
Plug that back into either original equation to solve for d.

To find the cost of each CD and DVD, we can set up a system of equations based on the given information.

Let's say the cost of one CD is 'x' dollars and the cost of one DVD is 'y' dollars.

From the first sentence, we know that:

13x + 7y = 99.80 (equation 1)

And from the second sentence, we know that:

12x + 7y = 84.11 (equation 2)

Now, we can solve this system of equations to find the values of x and y.

First, let's subtract equation 2 from equation 1:

(13x + 7y) - (12x + 7y) = 99.80 - 84.11

This simplifies to:

x = 15.69

Now, we can substitute the value of x into either of the original equations to find the value of y. Let's use equation 1:

13(15.69) + 7y = 99.80

203.97 + 7y = 99.80

Subtracting 203.97 from both sides:

7y = -104.17

Dividing both sides by 7:

y = -14.88

Now we have the values of x and y, which represent the cost of one CD and one DVD, respectively.

Therefore, the cost of each CD is $15.69 and the cost of each DVD is -$14.88. Note that a negative cost for a DVD is not possible in this context, so there might be an error in the given information or calculations.