If the original length and breadth of the rectangle/parallelogram was L and B respectively, then
Original Area = L*B
Once the dimensions are doubled, the length and breadth are now 2L and 2B respectively, so
New Area = 2L*2B = 4LB
New Area/Original Area = 4LB/LB = 4.
So, the new area is 4 times bigger. Therefore the original area is one fourth of 20cm², which is 5cm².