To solve this problem,
you can use the given values for x and y to calculate the net percentage change in value.
Here, x = 20 and y = -20.
Therefore, the net percentage change in value is:
(x + y + xy/100)%
= (20 - 20 + (20 * -20)/100)%
= (-4)%
Since the sign is negative, there is a decrease in value by 4%