The diagonal of a rectangle is sqrt(41) cm. and its area is 20 sq. cm. The perimeter of the rectangle must be:

$l2+b2$ =  $diagonal2$=40

Also, lb=20

$l+b2=l2+b2+2lb$ = 41 + 40 =81 

(l + b) = 9.

Perimeter = 2(l + b) = 18 cm.