A rectangular tank whose base measures 4 m by 3 m holds 24000 litres of water when filled to the brim. Find its depth in metres.

Since 1000 litres = $1{\text{m}}^3$, the tank holds $24000/1000=24{\text{m}}^3$. Its base covers $4\times 3=12{\text{m}}^2$, so the depth = 24/12 = 2 m. (Dividing the 24000 litres straight by $12{\text{m}}^2$ gives an absurd 2000 m; dividing by 1000 twice turns the capacity into $0.024{\text{m}}^3$ and gives 0.002 m; 20 slips the conversion by a factor of ten.)