Since the number of photons is conserved
(one photon emitted for every one that is absorbed) the number of
photons in the initial beam equals the number transmitted plus the number
fluoresced.
In part a) the number of photons transmitted was found to be 2.09 x
10^{17}.

So, the number of photons fluoresced is:

1.0 x 10^{18} photons - 2.09 x 10^{17} photons

= 7.91 x 10^{17} photons

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