How did they calculate the Max debt and Min debt as well as the WACC?  

According to the rating category, if Deluxe tends to maintain at AA rating. The Long-term debt/capital (%) should between -1.1% and 21.1%. So we calculate maximum and minimum bond issued as following: Max debt Min debt AA 61.33 (13.01) Singh wants to lower the cost of capital as well. Because the cost of debt is lower than the cost of equity. The more debt, the lower the cost of capital. Therefore we get lowest WACC of 7.05%. Cost of debt(pretax) AA 5.50% Tax rate 38% Cost of equity 10.35% Debt 0.48 Equity 0.52 WACC 7.05%