An irrigation channel 200 m long has the same cross-section throughout: a trapezium 4 m wide at the top, 2 m wide at the bottom, and 1.5 m deep. What is the full capacity of the channel, in cubic metres?

A solid of constant cross-section holds (area of cross-section) $x$ length. The trapezium measures $(1/2)\times (4+2)\times 1.5=4.5$ sq m, so the capacity = $4.5\times 200=900$ cubic m. The trap 1800 drops the 1/2 from the trapezium formula; 1200 uses the top width alone, $4\times 1.5=6$ sq m, treating the channel as a rectangular trough; 450 halves the correct area a second time.