The volume that is needed to built by the bricks
= Volume of outer dimensions of sump - Volume of sump
= 6.2 x 5.2 x 4.2 - 6 x 5 x 4
= 135.408 - 120 = 15.405 m3
Volume of one brick = 20/100 x 10/100 x 5/100 = 0.01 m3
∴ Total number of bricks required to build the sump
= 15.408 / 0.001 = 15408