**6900 bricks**

Logic : Dimension of cubical room is 800*600*400 (Converted to cm)

Size of brick used : 20*10*8

So, first side we can use (800/20)*(400/8) bricks. that is, 2000 bricks.

The opposite side of this wall we can use the same, that is 2000 bricks.

hence the other to sides length is now decremented by 20. that is 580.

these side we can use {(580/20)*(400/8)}*2 bricks. that is 2900 bricks.

Then the total number of bricks used = 6900 .