25 is 32% of what number?

You would set up the proportion 25/X= 32/100. 
25*100 is 2500, 32*x is 32x
2500 = 32x
/32       /32   dividing by 32 on both sides
78.125 = x

You can solve that by cross multiplication.
If 25 is 32%, then x is 100%:
25/32 = x/100
25*100 = 32*x
x = 25*100/32 = 78.125

To check the solution: 78.125*(32/100)=25

For this kind of question you just have to multiply times 100/percentage, that's what you always arrive to with the cross multiplication.
So if they asked you, 50 is 25% of what? You'd do 50*(100/25) = 200.
It's the reverse of finding the percentage of a number.