Adding or subtracting a percentage doesn seem to work correctly?
A. You may be encountering a common calculator special case. If there is an immediately pending addition or subtraction operation then the normally computed percentage is multiplied by the initial term of the addition or subtraction. This calculates the percentage rise or percentage fall of the initial term. For example 120 + 10% has a calculated percentage rise of 12 and an overall result of 132 (120 + 12).