2 . 一种袋装食品用生产线自动装填，每袋质量大约为50g，但由于某些原因，每袋食品不会恰好是50g．现随机抽取100袋食品，测得的质量数据如下：

单位：g

57          46          49          54          55          58          49          61          51          49 51          60          52          54          51          55          60          56          47          47 53          51          48          53          50          52          40          45          57          53 52          51          46          48          47          53          47          53          44          47 50          52          53          47          45          48          54          52          48          46 49          52          59          53          50          43          53          46          57          49 49          44          57          52          42          49          43          47          46          48 51          59          45          45          46          52          55          47          49          50 54          47          48          44          57          47          53          58          52          48 55          53          57          49          56          56          57          53          41          48

(1)为了获得样本数据的分布情况，试制作频率分布表；
(2)绘制频率分布直方图及频率分布折线图．

3 . 校高一年级有男生180人，女生120人．某统计小组为调查本年级学生身高情况，采取分层抽样的方法从总体中随机抽取样本，其中男生抽取18人，女生抽取12人．将男生组看作样本，计算出样本的平均身高为173.5cm，方差为17；将女生组看作样本，计算出样本的平均身高为164.0cm，方差为30．试根据以上数据计算由，组成的样本的方差，并估计总体方差．