Randy has w blocks. He uses x blocks to build a tower and y blocks to build a house. How many more blocks did he use to build the tower than he did to build the house?
The number of blocks Randy used to build the tower is x. The number of blocks Randy used to build the house is y. The number of blocks Randy used to build the tower minus the number of blocks Randy used to build the house is x - y. Therefore, the answer to the question is x - y = 33 - 13 = 20.