Let's break down the problem step by step.

Given:

• The cost of a regular size Twix is $4.
• The cost of a king size Twix is $4.50.
• The total cost involves r regular-sized Twix and k king-sized Twix.

To write an expression for the total cost:

• For r regular-sized Twix, the cost will be $4r$ (since each regular size costs $4).
• For k king-sized Twix, the cost will be $4.5k$ (since each king size costs $4.50).

Thus, the total cost expression would be:

$4r + 4.5k$

Let's check the options:

• Option 1: $4.5k + 4r$ — This matches the correct expression.
• Option 2: $4 + k + r + 4.5$ — Incorrect, as it adds up terms incorrectly.
• Option 3: $4l + 4.5r$ — Uses incorrect variables.
• Option 4: $-4r - 4.5k$ — Incorrect signs (negative instead of positive).

So, the correct answer is Option 1: $4.5k + 4r$.

Would you like more details on the steps? Here are 5 follow-up questions to deepen your understanding:

1. How do you determine the cost for multiple items using variables?
2. What is the role of coefficients in such expressions?
3. How do you interpret negative signs in cost expressions?
4. How would the expression change if the costs for regular and king sizes were different?
5. What is the significance of variable substitution in solving word problems?

Tip:

When solving word problems, always define your variables clearly and match them with the given information.