The book uses average flow and does not solve a velocity profile. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
This flow is an average-flow ledger
The checked claim is only 3 times 4 equals 12.
3⋅4=12
Do not infer a velocity profile
The diagram checks one average speed; it does not assert a parabolic profile or pressure law.