So the question is 25% of what number is 21. Lets turn that into an equation: let x be the number we are looking for. We need to take 25% from it and we know that that is equal to 21. Mathematically: 25% * x= 21. Or: (25/100) * x = 21. When we multiply both sides with 100 and divide both sides by 25 we get: x=21*(100/25). And we get that x=84.