To express the relationship where the cost of ice cream custard is directly proportional to its weight, 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 know the value of \( k \). For example, if the cost is $2 per ounce, then:
\[ C = 2w \]
In this case, 2 would be the constant of proportionality. If you have a specific cost per ounce from the problem or an example, you can substitute that value for \( k \).