In Olympic Stadium, meanwhile, America's top female sprinters outpaced their Jamaican competitors to earn gold in the prestigious 4 x 100-meter relay -- though their long dominant countrymen fell just short in the 4 X 400-meter men's race.

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Google founders Sergey Brin and Larry Page are using company money to fund another X-Prize, to create a private race to the moon.

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Plus he still has a crew to race one of his five yachts in the more low-key X-40 series that will travel through England, Boston, Oman and elsewhere.

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But history on Saturday also favored the leaders, as the most decorated Olympian of all time tallied another win with his American teammates in the 4 x 100-meter medley relay, handing Michael Phelps his 18th gold medal in the last race of his storied swimming career.

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The final day included a tour of the Environmental Protection Agency testing facility in Ann Arbor, and at the Michigan International Speedway, where the Progressive Insurance Automotive X PRIZE competition teams were testing their self-made, eco-friendly cars right on the race track.

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