Answer: There is none.
Explanation:
There is no proportional relationship between Celsius and Fahrenheit because for a proportional relationship to exist, the two variables need to have an equivalent ratio between them which would ensure that their values vary constantly when they are either divided or multiplied.
In other words, only the arithmetic operations of multiplication and division should be present. In the above there is an additive operation which renders the two variables being Fahrenheit and Celsius, non-proportional.