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.

Also, lb = 20.

(l + b)² = (l² + b²) + 2lb = 41 + 40 = 81

 (l + b) = 9.

 Perimeter = 2(l + b) = 18 cm.