If $P(A\cup B)=0.8andP(A\cap B)=0.3$ . Then $P\left(\overline{A}\right)+P\left(\overline{B}\right)$                                  

$P\left(\overline{A}\right)+P\left(\overline{B}\right)$  =$2-\left[P\left(A\right)+P\left(B\right)\right]$

                            =$2-\left[P\left(A\cup B\right)+P\left(A\cap B\right)\right]$

                            =$0.9$