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 1017.

So, the number of photons fluoresced is:

1.0 x 1018 photons - 2.09 x 1017 photons

= 7.91 x 1017 photons

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