WASHINGTON: It’s towards the end of our interview that Bill Gates reveals new numbers on how much his charitable Foundation has now spent in its efforts to combat preventable diseases and reduce poverty.

“I’ve given over 100 billion,” he says, “but I still have more to give.”That’s dollars, just to clarify, worth about £80bn.It’s roughly equivalent to the size of the Bulgarian economy or the cost of building the whole HS2 line.

But to put it in context, it’s also around the same as just one year of Tesla sales. (Tesla owner Elon Musk is now the richest man on the planet, a position Gates held for many years.)The co-founder of Microsoft and his fellow philanthropist Warren Buffett are combining their billions through the Gates Foundation he originally set up with his now ex-wife Melinda.

Gates says philanthropy was instilled in him early on. His mother regularly told him “with wealth came the responsibility to give it away”.His Foundation’s 25th anniversary is in May, and Gates exclusively revealed the $100bn figure to the BBC.

He tells me, for his part, he enjoys giving his money away (and around $60 billion of his fortune has gone into the Foundation so far).When it comes to his day-to-day lifestyle, he doesn’t actually notice the difference: “I made no personal sacrifice. I didn’t order less hamburgers or less movies.” He can also, of course, still afford his private jet and his various huge houses.

He plans to give away “the vast majority” of his fortune, but tells me he has talked “a lot” with his three children about what might be the right amount to leave them.Will they be poor after he’s gone? I ask him. “They will not,” he replies with a quick smile, adding “in absolute, they’ll do well, in percentage terms it’s not a gigantic number”.

Gates is a maths guy and it shows. At Lakeside School in Seattle, in eighth grade, he competed in a four-state regional maths exam and did so well that, at 13, he was one of the best high school maths students of any age in the region.