For successive increase / decrease in percentages, for successive years / times, the following formula will come handy:
Let the successive increase in percentages be a% and b%.
In that case, the total increase will be (a + b + ab/100 )%
Lets try an example.
If the increase is 10% and 20 %, the successive increase will be (10 + 20 + 20 * 10/100) = 32 %.
If there's an increase and a decrease, in that case, the decrease will be considered a negative value.
Lets try an example.
If there's an increase of 20% and then a decrease of 10%, the successive percentage will be (20 + (-10) + 20 * (-10) / 100 ) = 20 - 10 - 2 = 8% increase.
Now in case of discounts, the value of discount percentages will be considered negative.
Lets try an example
If Kouton's give 50% + 50% off on independence day, what is the final discount given by Koutons
The discount percentage will be (-50 + (-50) + (-50) * (-50) / 100 ) = -100+25 = 75% discount.
This method is useful when the percentage increase / decrease if for lot many times.
Once this method is mastered, successive percentage calculation will be a cakewalk and would relieve you of the pain of picking up pencil and scribing on paper..
Hope the methods helps you all.