Answer
$P(1~to~4~months)=0.415$
Work Step by Step
$P(1~to~4~months)=P(1~to~2~months~or~2~to~4~months)$
1) not a single event
2) OR
3) disjoint events
Use the Addition Rule for Disjoint Events (page 270):
From (a): $P(1~to~2~months)=0.19$ and $P(2~to~4~months)=0.225$
$P(1~to~4~months)=P(1~to~2~months~or~2~to~4~months)=P(1~to~2~months)+P(2~to~4~months)=0.19+0.225=0.415$
