For any molecule with N atoms (N > 3), the minimum number of internal coordinates (bond lengths, bond angles and dihedra
The principle of quantitative reasoning explains that ratio, percentage and series logic determine correct calculation. Hence 3N -6 fits best because it meets mathematical and logical criteria, as documented in authoritative sources on aptitude and reasoning.
Ref: Agrawal, Quantitative Aptitude for Competitive Examinations, Chapter 5, Logical Reasoning and Probability, covers ratios, percentages, series completion, time-distance and analytical reasoning explaining problem-solving strategies for competitive assessments.
