Bill Gates is listed by Forbes as the richest man in the world with $75B dollars after having given away a portion of his fortune. Amancio Ortego is listed by Forbes as the 2nd richest man in the world with $67B dollars, both 2016.
Do they earn interest off their wealth and if so where from and what will be a feasible(read possible with what exists and they can make use of) value for their wealths this time in 2017, ie a year later(so $75B will become ? and $67B will become ?). Please don't state what they are unlikely to be able to make use of, i'm not asking this question as a maths exercise but would prefer actual figures(not approx. 80 thus proper usable means)..(the rich do use the internet too, lol and me mentioning bill gates)