Area of square =\( \Large (Side)^{2}=20^{2} \) = 400 sq cm

Area of rectangle =\( \Large 1.8\times 400 \) = 720 sq cm

Let length and breadth of rectangle be 5x and x, respectively.

Then, according to the question.

\( \Large 5x\times x \)=720

=> \( \Large 5x^{2} \)=720 => \( \Large x^{2}=\frac{720}{5} \)=144

=\( \Large \sqrt{144} \)=12 cm

Perimeter of rectangle = 2 (5x + x) = 12x

=\( \Large 12\times 12 \) = 144cm