Here the expression simplifies to t(t-4)(t-1). Then Greg mentions that t-4 is congruent to t-1. Can someone please explain how that ‘t-4’ is congruent to ‘t-1’? Is this dealing with modular arithmetic (i.e., is he referring to mod 4)?

They seem much harder recently, not in terms of trickery, but rather testing concepts I have never seen before on the GRE. Would it be Possible to ask Greg to explain them with a bit more additional detail, like 30 sec to 1 min longer? THank you!