Should we include scrap value as an inflow for ROCE calculation?

Backpay Co is considering investing $50,000 in a new machine. The machine will have scrap value of $10,000 at the end of its five-year life. It is expected that 20,000 units will be sold each year at a selling price of $3.00 per unit. Variable production costs are expected to be $1.65 per unit, while incremental fixed costs are expected to be $10,000 per year.

Question
2. What is the project’s return on capital employed, based on average investment?

The ROCE I calculated is 36.7% because I included the scrap value in year 5’s cash flow, then averaged all cashflows, but that is not the case in the solution. The rest of my calculation is correct but I am a bit confused as to why we shouldn’t treat scrap value as an inflow for year 5, and one more thing to confirm, we only consider investment as an outflow in year 0 for payback period, not for ROCE calculation right. Please let me know. Thanks in advance!

Solution:
The correct answer is 30%.

WORKING

Annual cash flow = (20,000 × 1.35) – $10,000 = $17,000

Annual depreciation = (50,000 – 10,000)/5 = $8,000

Annual operating profit = 17,000 – 8,000 = $9,000

Average investment = 1/2 (50,000 + 10,000) = $30,000

ROCE = 9,000/30,000 = 30%