What the author has written is absolutely correct. Read it thorougly once again. The interest was not charged on one rupee, but on the outstanding of 30,009 and the purchase of 37,000. Since the card holder has not cleared the entire due the interest was charged on the total outstanding plus the new purchases made. The Re 1 may make an interesting case study - but actually if you talk to the bank they can waive the interest paid. They have discretion to waive the same.
