Posted by Rachal on .
Frank has a 65% chance of receiving an A grade in geology, a 60% chance of receiving an A grade in mathematics, and a 77% chance of receiving an A grade in geology or mathematics (or both). Find the probability that he receives A grades in both geology and mathematics.
I get: P(geo and math) = P(geo) + P(math)  P(geo or math)
= 0.65 + 0.60  0.77
= 1.25  0.77
= 0.48

Math 
PsyDAG,
You need to make up your mind. Is the .77 for both or eitheror?
I would assume that the above data (.77) is not needed.
To find the probability of both/all events occurring you need to multiply the probability of the individual events.
.65 * .60 = .39