At 38, Elon Musk has been a co-founder of PayPal, which he and his partners sold to eBay for .5 billion; and rocket builder SpaceX, which aims to commercialize the launching of payloads into orbit. He is also an initial investor in electric-car pioneer Tesla Motors, where he designs cars in addition to guiding the business; and solar energy company SolarCity, which sells and services solar energy equipment. In a two-part interview, Musk discussed luck, innovation and the fundamentals of starting a business. The first half of that interview, conducted by Wharton management professor John Paul MacDuffie , appeared in Knowledge at Wharton on May 13. You can find it here , along with an article about his recent visit with University of Pennsylvania students. In the second half, arranged by Wharton Entrepreneurial Programs , he tells Knowledge at Wharton the story of his entrepreneurial beginnings and what he learned about the value of pinching pennies.…