शदन्तविंशतेश्च

Operational rule vidhi

The affix ड comes in the sense of 'this is surplus in it' after a Numeral ending in शत् and after विंशति with the elision of अत् and अति.

Word by word (padaccheda)

Left context शदन्तविंशतेः pañcamī · ekavacanaIndeclinable च avyaya

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

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Full reading

शदन्तविंशतेः च

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix ड comes in the sense of 'this is surplus in it' after a Numeral ending in शत् and after विंशति with the elision of अत् and अति।
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 ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.