Assume sides of the rectangle are a,b.
Area of the rectangle = axb
After 20 % increase in each sides
New length and width are 1.2a and 1.2b respectively.
New area of the rectangle = 1.2a x 1.2 b
= 1.44 ab
Percentage increase in the rectangle = (1.44 ab -ab)x100/ab
= 44 %