"This is not exactly a marriage of equals. Melinda is better educated than Bill, having graduated from Duke University with a BA (a double major in computer science and economics) and an MBA. Harvard's most celebrated dropout, Bill was awarded an honorary degree last June.

Melinda also outperforms him athletically. She runs once a week with a few friends - seven miles in an hour, a brisk pace - and tries to exercise five days a week. She has completed the Seattle marathon and climbed, with ropes and crampons, to the peak of 14,410-foot Mount Rainier.

As for Bill, Melinda says, "He's finally started to run in the last year." To give him credit, he is an aggressive tennis player and a decent golfer - sometimes playing with Melinda. Beyond that, though, running on the treadmill while watching DVDs three nights a week is all Bill can do to keep up with his fit wife.

Melinda also understands people better than he does, Bill admits. In fact, he uses her as a sounding board, sometimes for personnel matters at Microsoft (MSFT, Fortune 500). In 2000, when Steve Ballmer, with whom Bill has worked for 28 years, replaced him as CEO, Melinda helped ease the awkward transition. "Melinda and I would brainstorm about it," Bill says. "You always benefit from your key confidante telling you, 'You think so-and-so stepped on your toes? Well, maybe he didn't mean to. Maybe you're wrong.'" Says the couple's close friend Warren Buffett, who has known them since 1991: "Bill really needs her."

When it comes to investing their philanthropic assets, Melinda wields even greater influence. Early on she and Bill agreed to focus on a few areas of giving, choosing where to place their money by asking two questions: Which problems affect the most people? And which have been neglected in the past? While many philanthropists take the same tack, the Gateses, who love puzzles, apply particular rigor."

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