25 is an integer and so there is no sensible way of writing it as a fraction or mixed number. As a whole number it is simply 25, exactly as in the question.

As a whole number, you can write 25 percent as 25%. As a part of a whole number, you'd write 25% of it as .25% or one quarter -- 1/4, of the whole number.

25 million is written out as 25,000,000. Since one million has six zeros in it, you can either add six zeros to 25 or move the decimal point over six places to the right of 25. Either method will give you the same answer of 25,000,000.