[y3di]

The question is how can you use a biased coin to reproduce an unbiased coin flip.

In this article, they give the example of a coin that flips heads 70% of the time. Their method was to flip the coin twice. They only considered cases where heads was flipped once and tails was flipped once because these cases both had a 21% chance of happening. So both cases have the same chance of occurring. In order to differentiate between the two cases, you consider the first coin flip to be your result. So heads than tails is the same as getting a heads and tails than heads is the same as getting a tails.