A finite beam-energy claim cites irradiance, illuminated area, and time. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Beam energy uses irradiance, area, and time
The energy ledger cites a checked irradiance source.
E=IAt=6⋅3⋅4=72J
The illuminated area is a new source field
The illuminated surface area is separate from the original beam-area quotient.