概率论的题目，看不懂答案是什么意思以及它们的结果是怎么计算的，求讲解，谢谢。

Question

如题所述

Answer 1

1)(2/20)*0.2+(5/20)*0.4+(3/20)*0.7+(10/20)*0.8
=0.02+0.1+0.105+0.4
=0.625
2)
=0.1/0.625
=100/625
=4/25
2.
(0.3*0.8+0.7*0.2)=0.38
3.
1) P(X>=-m)=P(X