To find the equation of the proportional relationship between the cost of the roast beef and its weight, we need to define what each variable represents:
- Let \( x \) be the weight of the roast beef in pounds.
- Let \( y \) be the cost in dollars.
Since the cost is proportional to the weight, we can express this relationship as:
\[ y = kx \]
where \( k \) is the constant of proportionality (the cost per pound).
From the problem, Nigel paid $10 for 2 pounds of roast beef. So we can find \( k \) using the given information:
\[ y = 10 \text{ dollars} \] \[ x = 2 \text{ pounds} \]
Substituting into the equation:
\[ 10 = k \cdot 2 \]
To find \( k \):
\[ k = \frac{10}{2} = 5 \]
Thus, the linear equation representing the relationship between the cost and the weight of the roast beef is:
\[ y = 5x \]
So the correct equation for the graph of this proportional relationship is:
\[ y = 5x \]