The Negative Particle remains unchanged in a word which begins with eka and of this eka there is the augment aduk (ad),
Thus एकेन न विंशतिः = एकान्नविंशतिः, एकान्नत्रिंशत् ॥ These are Instrumental compounds. The augment आदुक् is added to the final of the first member, so that we have optionally two forms एकात् न विंशतिः and एकान् न विंशति by 8.4.45.,