英语不说the room of Mary, 要改为Mary's room.这是中国学生常见的错。你的第二个可以写为:2. Mary's room is twice that of Tom's. 我给你几个例句:1. Its overall area is roughly twice that of Norway. 2. Black infant mortality is twice that of white babies. 3. For example, the uptake of silicon is about twice that of nitrogen in rice.
你可以把twice that of看作是习惯用法。