1. An apparatus, comprising: one or more processors; a storage in communication with said one or more processors; a display in communication with said one or more processors and with said storage, said display comprising a graphical user interface (GUI) configured for: displaying on said display any of four layers, said four layers comprising: a model layer; an equivalence layer; a fractions layer; and a sentence layer; any of: hiding and showing any layer of said four layers; linking and unlinking any two layers of said four layers; and locking and unlocking any portion of any of said four layers; and providing more than one type of model in said model layer.