100. The amount of any pension shall, from the 1st of January 1969, be adjusted annually, in the manner and at the time prescribed in accordance with section 119 of the Act respecting the Québec Pension Plan (chapter R-9) for the adjustment of the benefits payable under the said act, so that the amount payable for a month in any year following the first is equal to the product obtained by multiplying the amount that would have been otherwise payable for that month by the ratio that the Pension Index for that following year bears to the Pension Index for the year preceding that following year.