(3)In cases coming under clause (b) of sub-section (2), where the income liable to tax cannot be definitely ascertained, or can be ascertained only with an amount of trouble and expense to the assessee which in the opinion of the Board is unreasonable, the rules made under this section may-(a)prescribe methods by which an estimate of such income may be made; and(b)in cases coming under sub-clause (i) of clause (b) of sub-section (2) specify the proportion of the income which shall be deemed to be income liable to tax, and an assessment based on such estimate or proportion shall be deemed to be duly made in accordance with the provisions of this Act.
