Question

The A.P. Statistics exam consists of a multiple-choice section where each question has five choices. Sara estimates that she has a 75% chance of knowing each question. If she does not know the answer, she will randomly guess.

a) What is the probability of Sara getting a question correct?

b) The A.P. exam has 40 questions, how many might Sara expect to get correct?

Answer 1

It's more than 75% because if she knows it that's 75% already, then you add 20% of the 25% she didn't know.
That's .20 • .25 = 0.05 or 5%.
She actually has an 80% chance. 
Then 0.80 • 40