Cenvat credit reversal required where common inputs serve dutiable and exempted goods; prescribed formula must be applied. Rule 6 of the Cenvat Credit Rules, 2002 requires that where a manufacturer has not maintained separate inventory and has taken credit on common inputs used for both dutiable and exempted products (except as in sub rule (3)(a)), the prescribed reversal in respect of exempted goods must be made; there is no option to reverse the actual input credit. Failure to pay the prescribed amount will attract recovery with interest under Rule 12 and application of provisions of Rule 13.