ROI and RI

A division currently earns a return on investment (ROI) of 20%. It is considering investing in a project which has a residual income (RI) of $1,000 at an imputed interest charge of 20%.
What is the effect on the division’s ROI if the project is undertaken?

Increase
Decrease
Remain the same
No possible to tell from this information

A useful way of answering many ratio analysis questions is to substitute some simple
numbers into the problem. For example, if the division currently earns an ROI (operating profit
over net assets) of 20%, this could be represented by operating profit of $20,000 and net assets
of $100,000. Residual income is calculated by operating profit – (net assets x imputed interest
rate). A residual income of $1,000 could be represented by an operating profit of $11,000 less
an imputed interest charge of $50,000 x 20%. Therefore the new ROI would become (existing
operating profit + project operating profit) ÷ (existing net assets + project net assets) =
($20,000 + $11,000) ÷ ($100,000 + $20,000) = 25.83% = an increase in ROI. Far quicker
though is to realise that a project offering a positive residual income at an imputed interest rate of
20% must be offering a return higher than 20%, and therefore must improve the existing ROI of
20%.

Sir, I thought for a while and understood what they are trying to explain, but it seems to me that project net assets in their equation is $50,000, instead of $20,000?? Therefore,

(existing operating profit + project operating profit) ÷ (existing net assets + project net assets) =
($20,000 + $11,000) ÷ ($100,000 + $50,000) = 20.67%