My understanding of why V1, Vr and V2 are all 84.2 knots in a DC-3 is that the plane did not have these numbers in 1936. At some point (maybe 1950s?) it was decided to impose them them upon it. Instead of of doing new flight testing on an already ancient airframe, they worked the regulatory math (can not be less than 1.2) off the stall speed which is based on max gross takeoff weight. Later on it was decided to change the speeds from mph to knots. That math brought about the 84.2 knots.
Yes, it was an oral question.