وَعَلَى هَذَا يَتَغَيَّرُ الْفَرْضُ فِي كُلِّ عَشْرٍ مِنْ تَبِيعٍ إِلَى مُسِنَّةٍ.
Translation: On this basis, the prescribed due amount changes at each ten-cattle increment from tabi‘ to musinnah.
The thirty- and forty-animal units underpin the higher brackets: at 60, two tabi‘; at 70, one musinnah and one tabi‘; at 80, two musinnah; at 90, three tabi‘; at 100, two tabi‘ and one musinnah.
The obligation changes not because the age of every due animal must continuously increase, but because the number and combination of tabi‘ and musinnah alter with the valid partition into 30s and 40s.