Effective duty payable must be deducted from cum duty price to determine assessable value under central excise. Deduction for assessable value under section 4 must be the effective duty payable on the goods, after taking exemption notifications into account; tariff-rate or notional duty is not deductible. The Finance Act, 1982 amendment (retrospective to 1-10-1975) mandates this rule, and assessing practices that deducted notional duty should be corrected and excess collections regularised per earlier Ministry circulars.