If you gave AMF a million dollars, they would distribute an extra 200000 bed nets. If you gave them five dollars 200000 times, the total effect would be an extra 200000 bed nets. The average effect would be one bed net. If you don’t know AMF’s budget, and you can only guess to within a million dollars, then the expected effect of a five-dollar donation would be the average of all those possible five-dollar donations, which is one bed net.

If you gave AMF a million dollars, they would distribute an extra 200000 bed nets. If you gave them five dollars 200000 times, the total effect would be an extra 200000 bed nets. The average effect would be one bed net. If you don’t know AMF’s budget, and you can only guess to within a million dollars, then the expected effect of a five-dollar donation would be the average of all those possible five-dollar donations, which is one bed net.