The year’s basic pay totals ₱396,000.
Six months at ₱30,000 contribute ₱180,000. The next six at ₱36,000 contribute ₱216,000. Add them, then divide the total by 12.
Six months at each salary give ₱33,000 in 13th-month pay
Both eligible bonuses count toward the same ₱90,000 limit.
Here, ₱80,000 of 13th-month pay and an eligible ₱30,000 bonus total ₱110,000. The ₱20,000 above the limit is added to taxable income; it is not a ₱20,000 tax bill.



