#### Answer

No formula associated with this problem.

#### Work Step by Step

To start this problem, a probability tree should be drawn. The probability tree is attached below.
P(answering correctly) = P(knows and guesses correctly) + P(doesn't know and guesses correctly)
P(answering correctly) = (0.4)(1.0)+(0.6)(0.2)
P(answering correctly) = 0.4 + 0.12
P(answering correctly) = 0.52