A boundary wall 8 m long, 3 m high and 25 cm thick is to be built from bricks each measuring 25 cm by 12.5 cm by 7.5 cm. How many bricks are needed, assuming no mortar?

Work in centimetres throughout: the wall is $800\times 300\times$ 25 = 6000000 cubic cm and each brick is $25\times 12.5\times$ 7.5 = 2343.75 cubic cm, so the count = 6000000/2343.75 = 2560. The trap 768 compares the wall's FACE area 240000 sq cm with a brick's face 312.5 sq cm, which ignores how many bricks lie through the thickness; 256 reads the thickness as 25 mm rather than 25 cm; 25600 comes from a misplaced decimal point in the brick's volume, using 234.375.