Free will is one of the easiest hard questions, as millennia-old philosophical dilemmas go. Though this impossible question is generally considered fully and completely dissolved on Less Wrong, aspiring reductionists should try to solve it on their own.
The following posts can be read to set up the problem of "free will" and what constitutes a good solution from a reductionist perspective, without entirely giving away the solution. (When Yudkowsky wrote these posts, he thought he could get away with just leaving it as a practice problem, and some of the posts state that the problem will be left open. However Yudkowsky did eventually find that he needed to write out the whole solution.)
For spoiler posts see free will (solution).