Answer
The number of photons in a 1.00mJ burst of radiation is $1.636\times10^{15}$ photons.
Work Step by Step
Considering the formula: $$E=N\times E_{photon}$$
$E_{photon}$ : the energy of a photon
$N$ : the number of photons
$E$ : the total energy of all the photons
This formula shows that the total energy of all the photons are the total addition of all the energy of each photon, which is obvious by common sense.
In this case, we already know the total energy: $E=1.00mJ=10^{-3}J$.
The energy of a single photon is taken from part a): $E_{photon}\approx6.112\times10^{-19}J$.
Therefore, the number of photons in a $1.00mJ$ burst of radiation is:
$N=\frac{E}{E_{photon}}=\frac{10^{-3}}{6.112\times10^{-19}}\approx1.636\times10^{15}$ photons
