The paper discusses the invariant aspects of a mental task representation by the solver. An experimental study was conducted with schoolchildren who solved the same textual algebra tasks with systematically varying demands. The results indicated that the subjects regardless of their competence level practically did not pay attention to the demand variation. The invariants discovered in the equation-making algebra tasks suggest the existence of a phenomenon similar to the linguistic phenomenon of usage, that is, conventional usage of a language unit by all native speakers of a given language.