Are you sure there is a formula? I'm not sure there is sufficient information to be sure of an answer.

In particular, if we continue the table down each column, it appears that the value will reach zero when B = 20, which is an odd way to "prorate". But if we assume that to be true, then 67% = 2/3 does make sense, as we would be descending from 100 to 0 over three steps (50->40->30->20), so each step would reduce the amount by 1/3. And we could make a formula for that. But I won't do so without some reason to believe it makes sense to do that. After all, math is about truth, not about guesses.

What is the context of this question? Is it an exercise from a source that gives additional information, and that has taught something relevant; or is it just someone's random idea?