Determination of unpaid tax: administrative notice requires payment with interest and penalty unless promptly paid. Where the proper officer finds tax unpaid, short paid, erroneously refunded, or input tax credit wrongly availed for reasons other than fraud or wilful suppression, he shall serve a show cause notice requiring payment of the specified tax with interest under section 50 and a penalty; a later statement on same grounds is deemed notice. Voluntary payment of tax and interest before notice prevents proceedings on that amount unless short, and payment within thirty days of show cause notice avoids penalty. The officer determines tax, interest and prescribed penalty after representations and must order within the statutory limitation period.