你是想表達：在你們班級,女孩是男孩的兩倍是吧.但是你上面的表達都沒有謂語,這應該是個完整的句子才對.
有以下表達：
in my class,girls are two times（or twice) as many as boys.
in my class,there are twice more girls than boys.
倍數表達：
1.times+形容詞（副詞）比較級+than
eg:This hall is five times bigger than our classroom.
2.times+as+形容詞（或much)或副詞原級+as...
eg:The big box is four times as heavy as the small one.
3.times+the+名詞（size,height,weight,length,width)+of
eg:Your playground is 900 metres wide,three times the size of ours.