After Tax Rate (if you can please dont use excel)

3. David Abbot is buying a new house, and he is taking out a 30-year mortgage. David will borrow $300,000 from a bank, and to repay the loan he will make 360 monthly payments (principal and interest) of $1,200 per month over the next 30 years. David can deduct interest payments on his mortgage from his taxable income, and based on his income, David is in the 20% tax bracket. What is the after-tax interest rate that David is paying