1. The problem statement, all variables and given/known data By setting the total energy of photons to Mc^2, estimate the maximum number of photons that could be used to make a black hole of mass M. This is a thermal physics problem, in case anyone needs a context; we're discussing the entropy of a black hole. 2. Relevant equations I'm given the entropy for a black hole. I also know the Schwarzschild radius. 3. The attempt at a solution I'm just having trouble seeing where to start. Probably this is something very basic and I'm just missing it. Can someone just help me to start?