Under Cheng, Didi has expanded in just four years to 400 Chinese cities. The service lets users digitally hail and pay for taxis, private cars, limousines, and commuter buses. Cheng says 80 percent of all taxi drivers in China now use Didi to find passengers. So many people use the app, it can be difficult to get a cab during rush hour without it. Investors recently valued Didi at $35 billion, making it one of the most valuable private companies in the world. Uber, with operations in almost 500 cities on six continents, is worth $68 billion. ... Cheng was born in Jiangxi province, a landlocked region in eastern China famous for being the cradle of Mao Zedong’s Communist revolution. His father was a civil servant, his mother a mathematics teacher. He says he excelled at math in high school but during his college entrance exams neglected to turn over the last page of the test, leaving three questions blank. He got into the Beijing University of Chemical Technology, less prestigious than the upper-echelon schools. ... it turned out that Didi had a few advantages over the competition. Some were copying Uber’s U.S. strategy of working with limousine and town car chauffeurs. But there are far fewer black cars than yellow cabs in China. ... Instead of imitating competitors and giving away smartphones to drivers, an expensive proposition for a capital-strapped startup, they focused on providing their free app to younger drivers who already had phones and were likely to spread the word about Didi.