Watching what billionaires are doing is just step one of a larger investment process.

There are over 2,900 people in the world with a net worth of over $1 billion, according to the World's Billionaires List from Forbes . Some build wealth by running a business, and others just allow trust fund managers to build wealth for them. But many of the world's billionaires continue to grow their wealth through investing in stocks.

Among them are Warren Buffett, Bill Ackman, and David Tepper. Let's look at one stock each of these investors has bought recently.

1. Warren Buffett

Warren Buffett became CEO of Berkshire Hathaway through investing. And after taking over the company, he continued to invest in stocks with the company's cash. That's continued for decades now, allowing Buffett's net

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