Let's break down the problem and write the cost function for purchasing 5 or more CDs.

Cost Function:

1. Initial Purchase: The first 5 CDs cost $29.
2. Additional CDs: Each CD beyond the initial 5 costs $4 each.

If $x$ represents the number of CDs over 5, then the total number of CDs is $5 + x$.

For 5 CDs, the cost is fixed at $29.

For every additional CD (each of these is represented by $x$), the cost is $4x$ dollars.

Cost Function Expression:

The total cost $C$ as a function of $x$ is:

$C(x) = 29 + 4x$

where:

• $x$ is the number of CDs over 5.

Cost for 12 CDs:

Since 12 CDs include 5 CDs plus 7 more (over 5), $x = 12 - 5 = 7$.

Substituting $x = 7$ into the cost function:

$C(7) = 29 + 4 \times 7 = 29 + 28 = 57$

Thus, the cost of buying 12 CDs is $57.

Would you like further details or have any other questions?

Here are some related questions:

1. What is the cost for buying 8 CDs?
2. How many CDs can be purchased for $73?
3. What is the cost of buying 20 CDs?
4. If the price of additional CDs changes to $5 each, how does the cost function change?
5. How many additional CDs can be bought if the budget is $100?

Tip: Always ensure to subtract the initial 5 CDs from the total number to find $x$ correctly when calculating additional CDs.