Dmitri wants to cover the top and sides of the box shown with glass tiles that are 5 mm square. How many tiles does he need? Dimensions=20,15,9

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