Answer
$N = 3.32\times 10^{14}~photons$
Work Step by Step
We can find the energy of each photon:
$E = \frac{h~c}{\lambda}$
$E = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{550\times 10^{-9}~m}$
$E = 3.614\times 10^{-19}~J$
We can find the number of photons emitted in each flash:
$N = \frac{P~\Delta t}{E}$
$N = \frac{(0.0012~W)(0.100~s)}{3.614\times 10^{-19}~J}$
$N = 3.32\times 10^{14}~photons$
