Risk Analysis questions

#1
Hi, I'm new on this forum!
I have some question about risk analysis, I'm doing some exercices and I don't understand some points.

1) Limit of probability union
P(A)=0,5
P(B)=0,2
No data about dependence, independence or mutually exclusive.
P(A+B)=P(A)+P(B)-P(A)*P(B|A)
In general I can always says 0,5=<P(A+B)=<0,7 right?

2) Relation between reliability availability and maintenance
I have no data about component type (unreparable, reparable when fail or tested).
Availability=0.98
It's guaranteed a reliability of 0.98?

I think I need more data about component maintenance and component type (unreparable, reparable when fail or tested): thanks to this data and formula I can calculate the result.
The way of thinking is correct?

Thanks