How to correctly calculate the probability?

Question

Probability theory

1) a. According to some data, the probability of occurrence of case A is 15% and case B is 85%
b. According to other data, the probability of occurrence of case A is 80% and case B is 20%
What is the average probability of occurrence of case A? can you calculate?
2) What is better?
a person loses 100 rubles.
a. Lose 75 rubles for sure
b. with a probability of 75% to lose 100 rubles
3) What is better?
a person loses 100 rubles.
a. save 25 rubles exactly
b. with a probability of 25%, can you save 100 rubles
with the calculation (formulas)?
Thanks