Investment appraisal (discounted CF)

Sir in your ‘Npv’ lecture you taught us to take balancing allowance/ charge in the year of disposal. But Sir there is a Q in FM study text chapter4 ‘test your understanding 13’ in which (3 year project) it says tax is paid at the end of year and relief on tax depreciation is claimed as early as possible, and in the answer it calculates tax relief on tax allowable depreciation in 3rd year rather calculating balancing allowance/charge. I have seen your lecture but I don’t understand this particular question. Please help me