A sheet of glass measures 2.4 m by 1.25 m. Find its area in ${\text{cm}}^2$.

Turn the sides into centimetres first: 240 cm by 125 cm, so the area = $240\times 125=30000{\text{cm}}^2$. Checking the other way, the area is $2.4\times 1.25=3{\text{m}}^2$, and $1{\text{m}}^2=100\times 100=10000{\text{cm}}^2$, so $3{\text{m}}^2=30000{\text{cm}}^2$. (Answering 3 leaves the area in $m^2$; taking $1{\text{m}}^2$ as $100{\text{cm}}^2$ gives 300 and taking it as $1000{\text{cm}}^2$ gives 3000 — the factor is squared, so it is 10000.)