Cost of acquisition in your case is indexed cost of acquisition as on 1981. Sec 49(1) of the Income Tax Act is given below for your reference:-
Cost with reference to certain modes of acquisition.
49. (1) Where the capital asset became the property of the assessee—
(i) on any distribution of assets on the total or partial partition of a Hindu undivided family;
(ii) under a gift or will;
(iii) (a) by succession, inheritance or devolution, or
(b) on any distribution of assets on the dissolution of a firm, body of individuals, or other association of persons, where such dissolution had taken place at any time before the 1st day of April, 1987, or
(c) on any distribution of assets on the liquidation of a company, or
(d) under a transfer to a revocable or an irrevocable trust, or
(e) under any such transfer as is referred to in clause (iv) or clause (v) or clause (vi) or clause (via) 98[or clause (viaa) or clause (vica) or clause (vicb)] or clause (xiii) or clause (xiiib) or clause (xiv) of section 47;
(iv) such assessee being a Hindu undivided family, by the mode referred to in sub-section (2) of section 64 at any time after the 31st day of December, 1969,
the cost of acquisition of the asset shall be deemed to be the cost for which the previous owner of the property acquired it, as increased by the cost of any improvement of the assets incurred or borne by the previous owner or the assessee, as the case may be.
Explanation.—In this sub-section the expression "previous owner of the property" in relation to any capital asset owned by an assessee means the last previous owner of the capital asset who acquired it by a mode of acquisition other than that referred to in clause (i) or clause (ii) or clause (iii) or clause (iv) of this sub-section.