The problem with these two doctrines in reformed theolgy, is that we hold them then to be equal. So saying that predestination is the result of foreknowledge would not be accepted by Calvin for example.
Calvin seemed to foresee that there would be people that would argue that God "distinguishes among men according as he foresees what the merits of each will be"(Inst. III, 22, 1). Calvin, accordingly, writes against this notion, "by thus covering election with a veil of foreknowledge, they not only obscure it but feign that it has its origin elsewhere"(Inst. III, 22, 1). Calvin contests that this view of foreknowledge makes man God's co-worker in salvation, and implies that election is ratified only by man's consent. This is to make the gravest of errors because it suggests that man's will is superior to God's plan, or at the very least, implies God's plan is partially dependent on man(Inst. III, 24, 3). In refutation of this view, Calvin asserts that "this plan was founded upon his freely given mercy, without regard to human worth"(Inst. III, 21, 7 emphasis added). source: http://www.reformedtheology.ca/calvin.html
This being said that predestination is not foreknowledge in the part of God, but a act of his sovereign will over his creation.
But the Bible still present a free will, and there is the problem between these two doctrines. The analogy I heard the most about these two doctrinal statement is: They are like the two rails of the railroad. The never join, but you still need them both to ride the train. This illustration demonstrate how some reformed theologians hold theses views. They are both presented in the Bible and in the mind of God they do not conflict one another. It is not a clear cut answer or an air tight argument, but to a certain degree it is one that give justice to the whole teaching of the Bible and does not place our view of reality of God's view of reality.