Question
Evaluate $\text{P}(\text{A}\cap\text{B})\ \text{if}\ 2\text{P}(\text{A})=\text{P}(\text{B})=\frac{5}{13}\ \text{and}\ \text{P}(\text{A}|\text{B})=\frac{2}{5}.$

Answer

$\text{Given:}\ \ \ \ \ 2\text{P}\left(\text {A}\right)=\text{P}\left(\text{B}\right)=\frac{5}{13},\ \text{P}\left(\text{A}|\text{B}\right)=\frac{2}{5}$
$ \therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \text{P}\left(\text{A}\right)=\frac{5}{26}$
$\text{Now}\ \ \ \ \ \ \ \ \ \text{P}\left (\text{A}\cap\text{B}\right)=\text{P}\left(\text{B}|\text{A}\right).\text{P}\left(\text{B}\right)=\frac{2}{5}\times\frac{5}{13}=\frac {2}{13}$
$\text{And}\ \ \ \ \ \ \ \ \ \ \text{P}\left (\text{A}\cup\text{B}\right)=\text{P}\left(\text{A}\right)+\text{P}\left(\text{B}\right)-\text{P}\left(\text{A}\cap\text{B}\right)= \frac{5}{26}+\frac{5}{13}-\frac{2}{13}=\frac{11}{26}$

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