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

$l2+b2=41$ (or)    $l2+b2=41$  

Also,  $lb=20$

$l+b2=l2+b2+2lb$  = 41 + 40 = 81

(l + b) = 9. 

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