True, seems to be a class project going on somewhere and one of the long threads got some people going in the right direction.
I've noticed a number of different "subjects" appearing not long after this semester would have started that seem to be 100% related to each other and many of the posters are just seeking the end answer to a problem, or they want us to design the entire circuit for them.
What will these students say if the instructor asks them "and how did you arrive at that answer?"
This sounds like an assignment. Along with using a multiplexer to perform the function of an adder, this is a common school project.
Simply requesting a circuit is not really a way to complete your assignment. Elevators may be controlled by relays, discrete logic, or by some computer/microcontroller. You learn by doing the work.
You can make things start to fall into place by producing a flow chart that describes the function of the elevator.