Bill Gates has always been known as a hard worker and in a recent interview, revealed just how fanatical about his work he was in his 20s.

While speaking with the BBC’s Desert Island Discs, the 60-year-old billionaire and co-founder of Microsoft said that he actually memorized the license plates of his employees to monitor when they arrived and left the office.

“I had to be a little careful not to try and apply my standards to how hard they worked. You know, I knew everyone’s license plates so I could look out in the parking lot and see when did people come in, when were they leaving. Eventually I had to loosen up, as the company got to a reasonable size.”

Currently valued at $79.2 billion, Gates is the richest person in the world and has topped the annual list of the world’s wealthiest people on well over 20 occasions.

The vast majority of his wealth comes from Microsoft, which he founded in 1975 with school friend Paul Allen.

Certainly the most famous car Gates has owned is a Porsche 959. Prior to 1999, the car wasn’t legal to drive on U.S. streets but following the passing ‘Show and Display’ rule, it was granted an exemption from crash regulations, allowing Gates to fully enjoy its 2.8-liter twin-turbo six-cylinder engine.

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