A canal is 6 m wide and 2 m deep, and the water in it flows at 8 km per hour. In 30 minutes, how many hectares of land can it irrigate if 10 cm of standing water is required? (1 hectare = 10000 square metres.)

In half an hour the water travels $8\times 0.5=4$ km = 4000 m, so the discharge = $6\times 2\times$ 4000 = 48000 cubic m. Spread 10 cm = 0.1 m deep, it covers 48000/0.1 = 480000 sq m, which is 480000/10000 = 48 hectares. The trap 96 uses a full hour of flow instead of 30 minutes; 480 divides by 1000 rather than 10000 when converting to hectares; 4.8 takes the standing water as 1 m deep instead of 0.1 m.