Tom needs to lower a rope down 6 stories.  One story is 10 feet. The only rope being sold is 20 feet long but you lose 25% when lashing them together. How many pieces of rope will he need to buy? Let's think first. Chain of thought:
He needs 10 * 6 = 60 feet. He loses 20 * .25 = 5 feet each time. So he gets 20 - 5 = 15 feet from each piece. That means he needs 60 / 15 = 4 pieces of rope.
Therefore, the answer is 4.