If 15 is 25% of a value, what is that value?

In the given question it is already given that 15 is the 25% of a certain unknown number. Using these information’s the unknown number can be found easily.
Let us assume the unknown value to be = x
Then we can write the equation as
25x/100 = 15
25x = 15 * 100
25x = 1500
x = 1500/25
  = 60
So the unknown number is 60. Hope this is what you were looking for and the procedure of doing the problem is clear enough.