Chapter 13 - Counting and Probability - 13.3 Probability - 13.3 Asses Your Understanding - Page 866: 49

$0.3$

We know that by the addition rule of probabilities $P(A\cup B)=P(A)+P(B)-P(A\cap B)$ If $A$ and $B$ are mutually exclusive, then $P(A\cap B)=0.$ Substitute the given/known values into the formula above to obtain \begin{align*} 0.85&=0.6+P(B)-0.05\\ 0.85+0.05-0.6&=P(B)\\ 0.3&=P(B) \end{align*}