Bill Gates replies to Warren Buffett's request for an update on his $30 billion donation announced in 2006.

Ten years ago, when we first got word of your gift to the foundation, we were speechless. It was the biggest single gift anyone ever gave anybody for anything.

We knew we owed you a fantastic return on your investment.

Of course, philanthropy isn’t like business. We don’t have sales and profits to show you. There’s no share price to report. But there are numbers we watch closely to guide our work and measure our progress.