To find the equation representing the total charge in dollars, \( y \), for a delivery of \( x \) cookies, we first need to determine the cost per cookie.
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The fixed delivery fee is $5.
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Thus, the equation can be represented as: \[ y = mx + 5 \] where \( m \) is the cost per cookie and \( x \) is the number of cookies.
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From the problem, we have two scenarios:
- For 12 cookies, the total cost is $20.
- For 18 cookies, the total cost is $27.50.
We can set up two equations based on these conditions:
For 12 cookies: \[ 20 = 12m + 5 \] Simplifying this gives: \[ 20 - 5 = 12m \implies 15 = 12m \implies m = \frac{15}{12} = 1.25 \]
For 18 cookies: \[ 27.50 = 18m + 5 \] Simplifying this gives: \[ 27.50 - 5 = 18m \implies 22.50 = 18m \implies m = \frac{22.50}{18} = 1.25 \]
Since both calculations yield \( m = 1.25 \), we can confirm that the cost per cookie is indeed $1.25.
Now we can write the equation for the total charge: \[ y = 1.25x + 5 \]
Thus, the equation that represents the total charge in dollars, \( y \), for a delivery of \( x \) cookies is: \[ \boxed{y = 1.25x + 5} \]