After per-photon energy is known, the photon count multiplies it into a total energy budget. The total is exact only for the declared count and identical photon energy; real beams add number uncertainty, loss, spectral spread, detector calibration, and background subtraction.
Once per-photon energy is checked, the count multiplies it into a total budget. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
First compute the per-photon energy
At 3 hertz, the checked per-photon energy is 6 joules.
Eγ=6J
Photon count multiplies the energy budget
For 4 photons, the total energy is 4 times 6, which is 24 joules.