Bill Gates vows to drop off world's rich list

Billionaire Bill Gates has pledged again to give his wealth away

He added that he will eventually "drop off" the world's rich list.

He is currently the fourth richest man in the world

Microsoft co-founder announced he would make a $20bn (£17bn) donation to his philanthropic fund.

Bill said he had an "obligation" to return his resources to society.

It is not the first time he pledged to give way his wealth

He first pledged to give away his wealth in 2010 but his net worth has more than doubled since then.

He said "As I look to the future, I plan to give virtually all of my wealth to the foundation," he said. "I will move down and eventually off of the list of the world's richest people.