STU Corporation's financial statements reveal:

• Total assets: $1,400,000
• Total liabilities: $400,000
• Common stock: $600,000
• Additional paid-in capital: $250,000
• Retained earnings: $300,000
• Net income: $250,000
• Total revenues: $1,000,000

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