The Mills Lunch Shop prepares fresh take-out entrees each day. On Tuesday, 40 baked chicken dinners were prepared at a cost of $3.20 each. A 20% spoilage rate is anticipated. Mills Lunch Shop sells the dinners at an 80% markup based on cost. They decided to offer a $1.00-off coupon in a newspaper advertisement. What markdown percentage does this coupon represent? (Round to the nearest tenth of a percent.)