Use the information for the question(s) below. Gulf Oil issued $100 million in perpetual debt (at par) with an annual coupon of 7%. Gulf will pay interest only on this debt. Gulf's corporate tax rate is expected to be 21% for the foreseeable future.

6) Gulf's annual interest tax shield is closest to: A) $1.47 million. B) $4.20 million. C) $7.00 million. D) $40 million.

7) The present value of Gulf's annual interest tax shield is closest to: A) $4.2 million. B) $7.0 million. C) $21 million. D) $60 million.

8) RSR Metal currently has no debt and an equity cost of capital of 14%. Suppose that RSR decides to increase its leverage and maintain a market debt-to-value ratio of 1/2. Suppose RSR's debt cost of capital is 8% and its corporate tax rate is 21%. Assuming that RSR's pre-tax WACC remains constant, then with the addition of leverage its effective after-tax WACC will be closest to: A) 10.8%. B) 12.4%. C) 12.8%. D) 13.6%

Please don't use excel to answer all questions. Its confusing thank you. the answers are 6, A 7,C 8, D. i just want to know the calculation of it.