Section C Nexus Q in FM mock – to tax adjust the discount rate or not?

Hello there, I wonder if someone could help please re the Nexus question in the mock. I reduced the cost of capital by the tax rate when calculating the npv as tax was applied to the cashflows. However this was wrong and I can’t work out why? As I understood that if you apply tax to your cashflows, then you use the after tax cost of capital.

Thank you 🙂
Here is the scenario

Nexus PLC, a prominent player in the automobile industry, is in the process of planning to set up its own engine oil processing plant. The initial market survey incurred a cost of
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25,000andtheinvestmentforsettinguptheplantisestimatedtobe800,000.

This new project life is expected to be 4 years and the company expects scrap proceeds of $250,000 at the end of the project. Demand projections over the four years are as follows: