[LeifCarrotson]

8*20 is 160 billion, so your endowment's investment return is pretty optimistic.

More importantly, inflation and opportunity costs will take a big chunk out of your investment. Assuming consistent 2% exponential growth, and a civilization lasting millenia, that generous $8 billion you set aside today for budget year 3018 is worth about $20 in today's dollars.

http://www.in2013dollars.com/2018-dollars-in-3018?amount=20&...

[leetcrew]

I assume a withdrawal rate of 5% (slightly aggressive) and assume the endowment will be $150bn after the remainder of gates' assets are donated. I don't think thats terribly unrealistic. I certainly dont mean that they would spend $8bn nominal dollars each year; I am aware of inflation and the impact it has on returns.