Please provide formulas of how i can calculate this?

Based on the following information, what is the firm's weighted average cost of capital of the non-operating assets, WACC N ? The firm: Cost of debt, RD:5% Cost of equity, RS:20% Total market value of debt, D: $800 Total market value of equity, S: $800 Corporate tax rate, T: 40% Operating assets: Total market value, O: \$1,200 Cost of debt, ROD:3% Cost of equity, Ros:12% a. .233 b. .253 c. .218 d. 199 The correct answer is: .253