Such income tax and super tax shall be calculated at the rates in force for the financial year in which he is required to pay the tax, and shall bear to the total amount of income tax and super tax so calculated on the said. ' total income the same proportion as the amount of such inclusions bears to his total income or, in cases where under the provisions of sub section (1 ) of section 17 both income tax and super tax or super tax are chargeable with reference to the total world income, shall bear to the total amount of income tax and super tax which would have been payable on his total world income of the said previous year had it been his total income the same proportion as the amount of such inclusions bears to his total world income: 24 Provided that, where the previous year of the assessee in respect of any source of income ends after the 31st day of December and before the 30th day of April, the order in writing issued by the Income tax Officer requiring the payment of income tax and super tax on that source of income shall substitute for the four quarterly payments hereinbefore specified, three payments of equal amount to be made on the 15th day of September, the 15th day of December and the 15th day of March, respectively: (b) If the notice of demand issued under section 29 in pursuance of the order under clause (a) of this sub section is served after any of the dates on which the instalments specified therein are payable, the tax shall be payable in equal instalments on each of such of those dates as fall after the date of the service of the notice or demand, or in one sum on the 15th day of March if the notice is served after the 15th day of December.