transparency - Formula for finding out the result of a color after applying x% opacity to it?

I have 2 colors

1. #a1a2a4



2. black (#000000)


3. white (#ffffff)

Now I need to know how much opacity (in a percent) must I apply to the second color (black) against a background of the third color (white) to achieve a color "closest" (as determined by the eye) to the first color (#a1a2a4).

How do I go about doing it?