Bill Gates didn’t stick around Harvard for long—he dropped out after three semesters to start Microsoft in 1975. But he was there just long enough to leave a lasting impression. One of Gates’ former professors, applied mathematics expert Harry Lewis, remembers him as inquisitive and “mature” beyond his years. According to Lewis, Gates had a habit of questioning authority and tackling tough problems. “He always wanted a challenge,” Lewis, who has been teaching at Harvard since the 1970s, told The Times of London recently. “I wasn’t surprised when he dropped out—I just wish I’d invested in him.” Gates left Harvard to launch Microsoft at a time when software was still an emerging industry. The move paid off—he became a billionaire by 31. Today, Microsoft is worth nearly $3.2 trillion, and Gates has an estimated net worth of $108.7 billion, according to Forbes. If Lewis had put just $100 into Microsoft when it went public in 1986, that investment would be worth about $650,000 today...