几个英语句子i like this jacket better than that one,but it costs a

几个英语句子
i like this jacket better than that one,but it costs almost three times as much
为什么是as much ,不是倍数+as.as么,怎么省略了
on hearing the news ,the mother bursted into tears and rushed out of the room ,followed by a few other people .为什么不是following
I will brush my teeth when i am done .其中 i am done是什么用法,被动语态?
我把follow理解成进入了哎，还有2题
第三个如果是被动那句子怎么翻译