Every earlier flow-rate lesson in this book fixed average speed at 4 m/s and only ever scanned area (chapter five's flow-area-three-row-scan). This scan holds area at 3 m² fixed and checks three average speeds instead: 2 m/s gives flow 6 m³/s, 4 m/s reproduces the book's own checked average speed exactly at 12 m³/s, and 6 m/s gives flow 18 m³/s. Average speed is the other half of the flow-rate product, and it had never been scanned on its own.
Every earlier flow-rate lesson fixed average speed at four metres per second and scanned area; this scan holds area fixed and scans average speed itself. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Every earlier flow row held average speed fixed at four
Every prior flow-rate lesson in this book kept average speed at 4 metres per second and only ever varied area. Here area stays 3 square metres, but average speed itself drops to 2 metres per second, giving flow 6 cubic metres per second.
3⋅2=6
Average speed itself is a source factor, not just area
Area never changes across these rows. Average speed alone rescales the checked flow value, the same way area did in chapter five's own flow-area scan.
3⋅4=12
A faster average speed raises flow at the same area
At average speed 6 metres per second, the same area gives flow 18 cubic metres per second. Three rows separate average speed from the area factor already scanned in chapter five.