Suppose you are increasing 100 by 20% (X%) then 100 will become 120, an increase of 20 points. In this case if we want to decrease the value to 100, we need to remove 20 points from the original value. ie remove Y%(which is equal to 20) from 120
(Y*120)/100 = 20
Y=20*100/120 = 16.66%
here Y<x because 20 is a smaller part of 120 when compared to 100.
the simple logic here is ,If we are increasing on value in a product you have to decrease the other value proportionally to make the result constant.
I exam you can use this tip:
1) If one factor of a product is
increased by X% then the other factor should be
decreased by
(X/100+X)*100 %. It means when one factor of a product is increased by
(N/D) then the other factor is decreased by
(N/N+D) 2) If one factor of a product is
decreased by X% then the other factor should be
increased by
(X/100-X)*100 %. It means when one factor of a product is decreased by
(N/D) then the other factor is increased by
(N/N-D)