Firstly, we need to find the conversion factor (something like between Celcius <=> Fahrenheit relations):
increase of is equal to increase of value and it's obvious, that both scales start in different places, so we need to find the constant number, which will be added during conversion:
we can treat it as a function:
Of course I assumed, that the units are linear to values, in reality, they might look different. I mean, that grafically our function is a linear function, which might be any other one as well. I hope you kow what I mean.