Calculate the tip amount: Subtract the bill amount before the tip from the total amount he paid, including the tip.
Tip amount = 25.00 (total paid) - 21.00 (bill before tip) = 4.00
Divide the tip amount by the bill amount before the tip and multiply by 100% to express it as a percentage:
Tip percentage = (4.00 tip amount) ÷ (21.00 bill) × 100% ≈ 19%
Therefore, Chad left an approximate tip of 19%.