Travie McCoy and Bruno Marss hit Billionaire reached No 3 on both sides of the Atlantic last year. The irony of the lyrics isnt lost on either of us. They are, Gates chuckles, regularly used by his three children to poke fun at him. They have apparently also introduced him to the 'joys of Lady Gaga, 'but the 12-year-old is always worried about the nine-year-old listening to songs with bad words. So hes like, No Skip that one So I only know some Lady Gaga songs. At 55, he has graced the cover of Forbes magazine many times. As the co-founder, with Paul Allen, of Microsoft, he grew a 1975 back-room start-up into a software behemoth worth, at its peak, $400 billion. Oprah Winfrey is a close friend; the pair meets regularly and she has discussed signing his 'Giving Pledge to donate the bulk of her $2.7 billion estate to charity. And the Queen gave him an honorary knighthood back in 2005. 'The Billionaire song is what my kids tease me with, he says. 'They sing it to me. Its funny. Its probably just as well his children have a well-developed sense of humour. Gates is officially the second richest man in the world, only losing the No 1 spot to Mexican businessman Carlos Slim last year, after holding it for nearly two decades, on a technicality; he has given away $28 billion to charity, so is now personally worth 'only $56 billion. But Jennifer, 15, Rory, 12, and Phoebe, nine, arent going to inherit anything like that much. 'I dont think that amount of money would be good for them.He is determined that his family life should be as unaffected as possible by his fortune, and that he should be a hands-on father. MO