Section eighteen of the Finance Act, 1920 (which, as amended by subsequent enactments, provides, in the case of married persons, for a deduction of tax on one hundred and forty pounds, increased, in a case where the total income of the claimant includes earned income of his wife, by an amount equal to nine-tenths of that earned income or by forty-five pounds, whichever is the less,) shall have effect as if the words " eighty pounds " were substituted for the words " forty-five pounds. "