It was sponsored jointly by Google for Entrepreneurs, the company's set of programs and tools to support startups, and Virgin's not for profit foundation Virgin Unite.

Here are some of the most interesting tips and exchanges.

Their core advice for entrepreneurs

Branson: "If you've got a great idea that would improve people's lives, just do it."

Musk: "You want to be extra rigorous about making best possible thing you can, find everything that’s wrong with it and fix it. Seek negative feedback, particularly from friends"

On failure

Musk: "In all of the companies I’ve been involved with, there's always been a very difficult time. It's usually shortly after the beginning. Things seem optimistic, rosy, and exciting for the first 6 months to a year, then things start to go wrong. You encounter issues you didn’t expect, step on landmines. It's bad. Years 2 to 4 or 5 are usually quite difficult. A friend has a saying, it's 'eating glass and staring into the abyss.’