这道题其实没看懂

NO.PZ2015120604000074

B.

The joint probability of  having a good today' performance and a netural next day's performance is 0.70.

C.

The joint probability of  having a good today' performance and a netural next day's performance is 0.20.

解释：

A is correct

$P\left(AB\right)=P\left(A/B\right)\times P\left(B\right)$$$

A means neutral performance

B means good economy

Therefore, The joint probability of  having a good today' performance and a netural next day's performance is $0.4\times 0.2=0.08$

$$

烦请老师仔细讲一下具体是怎么计算的，这个运算规则我没搞明白是怎么用在实际题目里的