If I had 100 billion dollars to give away, I would first invest as much as it would take design and build of the necessary facilities to provide clean water and adequate sewage systmes in all human inhabited areas of the earth.

We often hear about how so many of the poor in third world nations should have access to better medications. Rarely does anyone comment on the fact that the water sources these people are forced to use causes rampant dysentary which makes it very difficult to remain healthy enough to tolerate/absorb any medications ingested. 

After dealing with the water facilites, I would use any funds left over to buy out businesses and industries in Africa that are owned by Americans and Europeans which could generate food and clothing. After buying these out, i would then field applications from locals who would hope to run these industries and grant complete ownership the businesses and any profits to them on the conditions that they allow some degree of business management oversight by paid business consultants (paid by me). This would hopefully contribute to establishing a modern economy in third world Africa that could be somewhat self-sustaining.