(3)For the purposes of clause (b) of sub-section (1), the gross income of [an intermediary] [Substituted by Act 20 of 1954.] from mines worked directly by him shall be the average annual gross income from such mines calculated on the same basis as that specified in subsection (2); and an amount equal to five per centum of the gross income so determined shall be deemed to be the net income from royalties which he might have derived in respect of such mines if he had leased them to another person.
