that和thatof用法
(1)Theamountofcoalwasthreetimesasmuchasthatofoil.
(2)ThenumberofcarsproducedinfactoryAwasthreetimesasmanyasthatin
factoryB.上面两句为什么一个为that另一个为thatof,还有就是他们分别指代什么,求大神们回答