Based on the way the question is phrased, I'm assuming you're looking for a solution in terms of n. (If that's not the case please let me know).
First, let's assign the other number a variable name. Here I'll use x. Now we can write and equation with our variables:
x - n = 25
Note that the equation needs to be written in the order x - n because we know n is the smaller number. If we had written n - x the equation would give us a negative number because x is bigger than n.
So now we simply solve the equation for x by adding n to both sides.
x = 25 + n
That is to say that the difference of two numbers where n is the smaller number will be 25 when the other number is n + 25.
Hope that helps!