If your answer is a negative number then this is a percentage increase. Your car insurance premium was 400 before the increase. If the previous value was negative 1 and the current value is positive 99 then this happens.
If the previous value was negative 1 and the current value is positive 99 then this happens.
If the percentage is negative it means the sales of the product have decreased. Your car insurance premium was 400 before the increase. Since zero is equal to zero each of the 0s you added together contributed to the sum equally. The one used by most statisticians is b a a where a is the absolute value of a a when a is negative.