1. The average monthly gross rent of any building or land in a slum or clearance area shall be one-sixtieth of the total gross rent actually accrued to the owner thereof during the period of five consecutive years preceding the due of notification under section 17. 2. In determining the average monthly gross rent the Competent Authority or such officer as may be empowered by it in this behalf may, for such determination hold nay local inquiry and obtain, if necessary, certified copies of such extracts from the property tax assessment books of the municipal or other local authority concerned showing the rental value of the land or building and such other information in this regard as may be deemed necessary by it or him. 3. The net average monthly income for the purposes of subsection (1) of section 20 shall be sixty per cent of the average monthly gross rent, determined in the manner laid down in para. 1. The remaining forty per cent of the average monthly income gross rent shall not be taken into consideration in determining the net average but shall be deducted in lieu of the property tax payable to a local authority, income tax, collection charges, bad debts, repairs and maintenance of the building and other incidental charges. 4. In case any land or building, or any part thereof, has remained unoccupied or no rent in respect thereof has accrued to the owner during the whole or any part of the said five years, the gross rent of such land or building or part thereof, shall be determined on the basis of the rent actually derived from a similar land or building in the vicinity of the land or building, as the case may be, during the period it remained vacant.