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