STU Corporation's financial statements reveal:

• Total assets: $1,000,000
• Total liabilities: $300,000
• Common stock: $400,000
• Additional paid-in capital: $150,000
• Retained earnings: $200,000
• Net income: $180,000
• Total revenues: $800,000

Calculate: a) Debt-to-equity ratio b) Dividend yield c) Return on investment d) Operating profit margin