The tax liability in case of companies is higher of tax computed as per the normal provisions and Sec 115JB .
As per the normal provisions tax payable will be nil because of the set off of past losses.
As per MAT book profit will be computed by deducting b/f loss(excluding depreciation) and unabsorbed depreciation which ever is lower as per the books of accounts from the net profit.
In your case:
Net profit: 500000
Less: Lower of B/F Loss 650000 and UD Nil as per books of accounts i,e 0
Book Profit u/s 115JB: 500000
Therefore tax will be paid as per the provisions of MAT i,e 18.5% of 500000 (Book Profit)
PS: I have assumed loss return is filled within the due date to avail the benefit of set off