This boils down to simple algebraic manipulation.

So one possible ODE (the simplest, in fact) is

first find the number of adults in the city then find the number of people who support power plant and if the precent adults in the city is 100% then take the number of people that support power plants and find out what precent of adults In the city are power plant supporters.
Yes it is a function because in a function all values of x have exactly one value of y.