#### Answer

$0.3$

#### Work Step by Step

The Addition Rule of Probabilities says that: $P(A\cup B)=P(A)+P(B)-P(A\cap B)$.
If A and B are mutually exclusive, then $P(A\cap B)=0$.
Solve for $P(B)$ in the formula above to obtain:
$P(A\cup B)=P(A)+P(B)-P(A\cap B)\\
P(A\cup B)-P(A)+P(A\cap B)=P(B)\\
P(B)=P(A\cup B)-P(A)+P(A\cap B)$
Substitute the given values to obtain:
$P(B)=0.85-0.6+0.05\\
P(B)=0.3$.