An Australian doctor who founded a group of GP practices has become a billionaire at 38.  

But he has done it not by finding gold in the black hills of Medicare but by selling burritos.  

Dr Sam Prince founded Zambrero – the global Mexican fast food chain – back in 2005, a year before graduating from medicine at Monash University. 

He was 21 years old at the time. He said he wanted to offer a menu that was healthier than the Mexican food he had been serving while waiting tables in his part-time jobs as a student.