How to know which part does the "in which" clause describe?
Solution 1:
In a comment, John Lawler wrote:
It clearly refers to the complement clause of Suppose:, specifically denoting the case in which the coin is known to be either biased or not biased, and those are the only two choices, and it is not known which choice is correct. In that case -- the case of the coin being either one or the other -- the conclusion follows. And that is the which.