God's will

Prayers are or are not answered. Why? Because it was God's will; thus, making the prayers a waste of time. Unless, prayers are able to change God's will. Since God knows the future, the future is predetermined since he already knows what will happen, regardless of your prayers. Or, is God so powerful that, if enough people pray, he can change the future that he already knows will occur. If this is so, then prayers can change the will of God, making prayers, and thus mankind, more powerful than God himself. Then, instead of being an all powerful, overseer, God is merely powerful tool being told what to do by the prayers of man.