Imputed interest charge

I watched your lectures. Notional interest is actually imputed interest sir?
There’s a question
Traceable residual income for a division is being calculated. The following information is available.
1)Head office requires a rate of return of 10% for the division.
2) Properties specifically relating to the division, which are managed by head office amount to $800,000
3) Other assets totalling $400,000 are controlled by the division.
What is the imputed interest charge to be used in the traceable residual income computation for the division?
Correct answer is 40,000
Sir i dont get the logic. Please explain me in simple words.