The affix ḍa comes in the sense of 'this is surplus in it' after a Numeral ending in śat and after viṃśati with the elision of at and ati,
The indicatory ड् causes elision of टि portion 6.4.142 &c. Thus विंशदधिका अस्मिञ्छते = त्रिंशं शतं, एकत्रिंशं शतं, एकचत्वारिंशं शतं ॥ This rule applies to abstract Numerals. Hence not here, गोत्रिंशदधिका अस्मिन् गोशते ॥
So also after विंशति; as विंशं शतं ॥
Vart:- So also after compounds ending in विंशति, as, एकविंशं शतं ॥ This also applies to pure Numbers. Therefore not here, गो विंशतिरधिका अस्मिन् गो शतं ॥,