We know that the shorter side of the rectangle is 1/2 of the longer side (because it's a square folded in half). To meet this condition and to have a perimeter of 24cm, we may assume that the dimension of the rectangle are 8cm x 4cm.
Now if we unfold it, we'll get a square of 8cm x 8cm. To find the are we just need to find 8², which is 64 cm².
