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

Just look outside and then you’ll see where the awnser is

The answer is why would a teacher ask that