Dmitri wants to cover the top and sides of the box shown with glass tiles that are 3 5 mm square how many tiles does he need 20 cm 15 cm

Lets split the box in pieces and then add:
top = 20*15 = 300
sidesA = 2*20*9 = 360, is multiplied  by 2 cause there are 2 sides like this
sidesB = 2*9*15 = 270, is multiplied  by 2 cause there are 2 sides like this
so in total we have:
total = 300 + 360 + 270 = 930 cm^2
if we express this quantity in mm^2 we have:
1cm² = 100 mm²total = 930*100 = 93000 mm^2
so if the tiles are 5 mm^2 we need:
total tiles = 93000/5 = 18600
we need 18600 tiles of 5 mm^2