To express the cost of ice cream custard as directly proportional to its weight in ounces, you can use the equation:
\[ C = k \cdot w \]
where:
- \( C \) is the cost of the ice cream custard,
- \( w \) is the weight in ounces,
- \( k \) is the constant of proportionality (the cost per ounce).
To complete the equation, you would need to determine the value of \( k \) based on the cost per ounce of the custard. For example, if the cost is $2 per ounce, then the equation would be:
\[ C = 2w \]
If you have a different cost per ounce, substitute that value for \( k \) accordingly.