Surface area of the 5mm square = 5*5 = 25 square mm.
Dimensions of 20, 15, 9
For the face of 20 by 15,
He would need = (20*15) / 25 = 12
Since the face are 2 , so he would need = 12* 2 = 24
For the face of 20 by 9,
He would need = (20*9) / 25 = 7.2, that is approximately 8
Since the face are 2 , so he would need = 8* 2 = 16
For the face of 9 by 15,
He would need = (9*15) / 25 = 5.4, that is approximately 6
Since the face are 2 , so he would need = 6* 2 = 12
Total = 24 + 16 + 12 = 52
So for covering up the cuboid of dimensions 20, 15, 9, he would need at least 52 tiles
