In reply to atanu.biswas:
It may help to think of
solve x before y; as
choose x before y;. See this post for a better explanation.
In your x before y case, there are only two possible choices for the value x. So x has a 50% chance of being 0, and a 50% chance of being 1. Since there is only one solution with x≡0, that solution has a probability of ½. There are four solutions with x≡1, so each solution there has a probability of ½*¼ = ⅛.