(c)Where the payment by way of gratuity is made in respect of past services of the assessee extending over a period of not less than fifteen years,-(i)the total income of the assessee in respect of each of the three previous years immediately preceding the relevant previous year shall be increased by an amount equal to one-third of the amount of the gratuity included in the total income of the relevant previous year, and the average rate of tax for each of the said three previous years shall be calculated as if the total income so increased were the total income of that previous year; and(ii)the average of the average rates of tax for the three previous years immediately preceding the relevant previous year, calculated in accordance with sub-clause (i), shall, for the purposes of clause (a), be the rate of tax determined under this clause.