Who said anything about T-Bills? I'm talking about plain old fashioned interest from having your money sitting in a bank account. Let's just say for sake or argument (and because the math is easy) that he was earning 5% interest.
5% of 56 Billion is $2.8 Billion. Since that's an annual percentage rage, that's $2.8 Billion a year. Now obviously there are taxes on that, and with that kind of cash you would want to buy private insurance, which would knock a couple of points off, but also keep in mind that he's probably getting closer to 7% - 8% because the amount is so high. I'm just talking about a checking account here ... if the money was actually invested instead of just sitting there, the return could be potentially greater, but make no mistake there is interest being earned on that money in some form or another and I guarantee you it's at LEAST 5%.