FedEx Corporation reported the following financial details for 2023. They had a total revenue of $100 billion, with a cost of goods sold amounting to $60 billion. Their operating expenses were $30 billion, and they incurred an interest expense of $3 billion and a tax expense of $5 billion. The total assets of the company stood at $180 billion while the total liabilities were $80 billion.

1. Calculate the gross profit margin.
2. Determine the operating profit margin.
3. Compute the interest coverage ratio.
4. Determine the return on equity (ROE).