[
  "Use a square of side 100 as the original shape for easy calculation.",
  "New length = 120 (100 + 20%), new width = 80 (100 - 20%). New area = 120 x 80 = 9600.",
  "Percentage change = (9600 - 10000)/10000 x 100 = -4%. The area decreases even though one increase cancels one decrease arithmetically."
]