318. Where a taxpayer receives a payment under a retirement plan to which he has contributed the investment income of which has already been exempted from taxation under the Income War Tax Act (Revised Statutes of Canada, 1927, chapter 97) by reason of an election of the trustee or corporation administering such plan, he may include in computing his income only the amount remaining after subtracting from the payment the greater of the following two proportions of the said payment:(a) that the aggregate of the amounts paid by him under the plan during the period of such exemption is to the aggregate of all the amounts paid by him under the plan, and
(b) that the aggregate of the amounts so paid by him under the plan during the period of exemption with simple interest at 3% per annum computed from the end of the year of the payment of each sum so paid to the beginning of the payment of the pension benefit is to the aggregate of all the amounts paid by him under the plan with simple interest computed at the same rate and in the same manner.