Assuming you are referring to a 50% tax rate on a $120 purchase, the tax should have been $60, which makes the total purchase cost $180. If Herschel made a mistake, it could have been due to a number of factors, such as a miscalculation, a misunderstanding of the tax rate, or a miscommunication about the total purchase cost. Without more context, it's hard to determine exactly what caused the mistake.