The problem with this is that you have to be extremely honest about just how much more you value yourself than a random African. In absolute terms, a dollar can do so much good in Africa that you’d go rather broke even if you “only” considered yourself as valuable as a hundred Africans. If you lie to yourself about your preferences, as people tend to do, you’ll quickly see your true preferences reveal themselves.
The problem with this is that you have to be extremely honest about just how much more you value yourself than a random African. In absolute terms, a dollar can do so much good in Africa that you’d go rather broke even if you “only” considered yourself as valuable as a hundred Africans. If you lie to yourself about your preferences, as people tend to do, you’ll quickly see your true preferences reveal themselves.