Changing area at fixed average speed makes volumetric flow scale visibly. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Two square metres at four metres per second gives eight
Average speed stays 4 m/s. Area 2 square metres gives flow 8 cubic metres per second.
2⋅4=8
Three square metres gives twelve
Area 3 square metres at the same speed gives flow 12 cubic metres per second.
3⋅4=12
Four square metres gives sixteen
Area 4 square metres gives flow 16. The diagram still asserts average flow only, not a profile.