To find the equation representing the proportional relationship between the cost of roast beef and its weight, we start by using the information given:
- Natural paid $10 for 2.5 pounds of roast beef.
We can set up the relationship as follows:
- Let \( C \) represent the cost (in dollars).
- Let \( W \) represent the weight (in pounds).
- The problem states that the cost is proportional to the weight, which can be written in the form: \[ C = kW \] where \( k \) is the constant of proportionality (price per pound).
Next, we need to find the value of \( k \):
- We know \( C = 10 \) when \( W = 2.5 \).
From the equation: \[ 10 = k \times 2.5 \]
To solve for \( k \): \[ k = \frac{10}{2.5} = 4 \]
Now that we have determined \( k \), we can write the equation that describes the relationship: \[ C = 4W \]
This equation states that the cost \( C \) in dollars is 4 times the weight \( W \) of the roast beef in pounds.