Spirit Airlines reported the following for 2023: total revenue of $40 million, cost of services of $28 million, operating expenses of $8 million, interest expense of $1 million, and tax expense of $1.5 million. The total assets were $50 million, and total liabilities were $20 million.

1. Calculate the operating profit margin.
2. Determine the net profit margin.
3. Compute the interest coverage ratio.
4. Determine the debt-to-equity ratio.