I think that the concept of 100 billion dollars at our disposal is hard to wrap your mind around. It is such a large sum that you could split it 200 ways and still be giving an incredible amount of money. I think a reasonable solution would be to research 100 or so charities that you sympathized with (and if that is too many then perhaps 50 or 25 charities) and split the money equally between them. the charities would still be getting an enormous sum, and your money would be helping a lot more people than if you just donated to one institution alone.