Water in a canal 6 m wide and 1.5 m deep flows at 10 km per hour. In 30 minutes, how many hectares can it irrigate if the standing water needed is 8 cm deep? Take 1 hectare = $10000{\text{m}}^2$.

In 30 minutes the water advances 5 km = 5000 $m$, so the volume delivered = $6\times 1.5\times 5000=45000{\text{m}}^3$. Spread 8 cm = 0.08 $m$ deep it covers $45000/0.08=562500{\text{m}}^2$, and 562500/10000 = 56.25 hectares. (Using the full 10 km, i.e. a whole hour, doubles it to 112.5; reading 8 cm as 0.8 $m$ gives 5.625; 562.5 forgets to convert $m^2$ into hectares.)