In the proposed solution, the examiner calculates Cost of Debt using IRR approach. Tax relief (1-T) is not taken into account. Even, explicitly it is written Cost of debt is 6.48% (which should be the Return to Investors). However, after that in WACC calculation tax relief is considered.
In your lectures, you explained very thoroughly this topic. I assume that we should take into account tax relief (1-T), at the time when we calculate IRR (Cost of debt) for redeemable debt?
Strictly, what the examiner has done is wrong in that because it is redeemable debt he should have calculated the IRR of the after-tax flows. The examiner accepts this and made it clear that you would have got full marks for calculating the IRR properly.