Solution :
We know,
[tex]$\text{WACC}=\text{equity weightage } \times \text{equity cost} + \text{net debt weightage} \times \text{debt cost} \times (1 -\text{tax rate})$[/tex]
Net debt = debt market value - excess cash
= 110 - 10
= 100 million dollar
[tex]$\text{net debt weightage}=\frac{\text{market value of net debt}}{\text{equity market value+ net debt market value}}$[/tex]
[tex]$=\frac{100}{300+100}$[/tex]
= 0.25
[tex]$\text{equity weightage}=\frac{\text{market value of equity}}{\text{equity market value+ net debt market value}}$[/tex]
[tex]$=\frac{300}{300+100}$[/tex]
= 0.75
Therefore, WACC = 0.75 x 12% + 0.25 x 5% x (1 - 31%)
= 0.75 x 12% + 0.25 x 5% x (1 - 0.31)
= 0.75 x 12% + 0.25 x 5% x 0.69
= 9% + 0.862%
= 9.862%