(c)Where the additional salary or additional family pension, as the case may be, relates to only one previous year, tax shall be calculated on the total income of the said previous year as increased by the additional salary or additional family pension, as if the total income so increased were the total income of the assessee, and the amount by which the tax so calculated exceeds the tax payable by the assessee in respect of the total income of the said previous year shall, for the purposes of clause (a), be taken to be the tax on the additional salary or additional family pension, under this clause.