Section 20. The Collector shall from thnc to time determine what persons are cliargeable under this Part, and tl

20. The Collector shall from thnc to time determine what persons are cliargeable under this Part, and tlie amount that every such person shall be assesscd in accordance with the said schcd~~le; ancl in making such assessment income exeinpted under section seven shall be treated as chargeable uncler t l ~ s Part. Collector to determine persons rhnrgeable. Computation 21. I n the case of a person for the first time whenassessee becoming chargeable under this Part mrithin tlie year becomes chargeable of assessnient, the computation shall be made accorclwithin year. k g to an average of his income and profits for such period as the Collector shall, under the circunistances, ciirect. within year. k g to an average of his income and profits for such