A wall 10 m long, 4 m high and 24 cm thick is built solid with bricks measuring 25 cm by 12 cm by 8 cm. How many bricks are used?

Work throughout in centimetres: the wall measures $1000\times 400\times 24=9600000{\text{cm}}^3$ and one brick is $25\times 12\times 8=2400{\text{cm}}^3$. Number of bricks = 9600000/2400 = 4000. (Mixing metres with centimetres is what produces the tenfold slips 400 and 40000 — the wall is $9.6{\text{m}}^3$ and a brick is $0.0024{\text{m}}^3$, which agrees: 9.6/0.0024 = 4000.)