STU Corporation's financial statements reveal:

• Total assets: $1,600,000
• Total liabilities: $450,000
• Common stock: $700,000
• Additional paid-in capital: $300,000
• Retained earnings: $250,000
• Net income: $300,000
• Total revenues: $1,200,000

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