Multiplying by 1-t is accounting for the tax saving on debt in order to calculate the cost of debt to the company i.e. the after tax cost of debt. (We only multiply by 1-t when it is irredeemable debt. If redeemable debt we should calculate the IRR of the after-tax flows instead, as I explain in my free lectures).
However in Tisa the question specifically says that the 4.5% is already the after tax cost of debt.