$f_j/n_j$

$t_1=2.5$

$t_2=3$

$t_3=3.5$

$t_4=4$

$t_5=4.5$

$t_6=5$

$t_7=5.5$

频率

0/20

1/20

3/20

5/20

6/20

10/20

16/20