وَفِي ثَمَانِينَ: مُسِنَّتَانِ،
Translation: At eighty cattle, two musinnah are due.
80 = 40 + 40; each forty-animal component calls for one musinnah, so two musinnah are due.
The number of due animals remains two from 70 to 80, but their required age categories change. Merely saying “two cattle” would omit the legally specified type.