the next hint
Cut the rope into two parts - 25 ft and 50 ft. Tie one end of the 25 feet piece to the tree and make a loop at the other end. Thread the 50 feet rope through the loop and fold the rope in half. The length of the resulting rope is 25 + 50/2 = 50 ft. Get down to the lower tree. Pull the rope out of the loop and you've now got 50 feet of rope to climb down with.