A firm has an unlevered beta of 1.10.Assume that the tax rate is 25%, the risk free rate is 3% and the expected return on the market is 9.5%.Determine the optimal capital structure for a company based on the following information:

I have been using a spreadsheet with formulas and cell references to solve the problem but am stuck.

Leverage (Debt/Assets) | Credit Rating | Yield to Maturity (Before Tax)

10% AAA 2.2%

20% AA 2.5%

30% AA -2.9%

40% A 3.6%

50% BBB 4.6%

60% BBB -5.75%

70% BB 6.95%

80% B -8.85%

90% CCC -11.0%