In this Act, unless the context otherwise requires,.

(30) "taxable turnover" means the turnover of all sales of a dealer during the prescribed period in any year, which remains after deducting therefrom,

(a) the turnover of sales not subject to tax under this Act;

(b) the turnover of goods declared exempt or under a notification under sub-section (2) of section 5, and

(c) in case of turnover of sales in relation to works contract, the charges towards labour, service and other like charges, and subject to such conditions as may be prescribed:

Provided that in the cases where the amount of charges towards labour, service and other like charges in such contract are not ascertainable from the terms and conditions of the contract, the amount of such charges shall be calculated in such manner as may be prescribed;