Question

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

Answer 1

You need to make up your mind. Is the .77 for both or either-or?

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