length of side of square = x

percent decrease = n

area of rectangle = length * width

length of rectangle = x + .25x = 1.25x ("25% increase")

width of rectangle = x - nx = x(1 - n) ("some percent decrease")

area of rectangle = area of square = x^2

So:

0.2 is 20%, so there needs to be a 20% decrease in the width.