$0.2$
Suppose $A$ and $B$ are disjoint with $P(A)=0.3$ and $P(B)=0.5$. What is $P(A'\cap B')$?
$0.2$
\[\begin{align}P(A'\cap B')&=P((A\cup B)'),\text{ De Morgan's law}\\
&=1-P(A\cup B),\text{ complement rule }\\
&=1-[P(A)+P(B)-P(A\cap B)],\text{ addition rule}\\
&=1-[P(A)+P(B)-P(\emptyset)],\text{ given disjoint}\\
&=1-[0.3+0.5-0]\\
&=0.2.\end{align}
\]