(a)In a case referred to in clause (b) of sub-rule (1), the tax payable by the assessee on his total income of the previous year in which the payment by way of gratuity is received (such previous year being hereafter in this sub-rule referred to as the relevant previous year) shall be reduced by the amount, if any, by which the tax on the amount of the gratuity included in the total income of the relevant previous year, calculated at the average rate of tax applicable to such total income, exceeds the tax on the amount of such gratuity, calculated at the rate of tax determined under clause (b) or, as the case may be, clause (c).