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.