probability answer as a common fraction

Sally is taking a test with 10 questions. Each answer is either True or False. She knows the correct answer for five of the questions and randomly guesses on the other five questions. What is the probability that she will have at least eight correct answers on the test? Express your answers as a common fraction.

I thought the answer was 3/5 but it was marked wrong. Would you multiply 5/10 by 3/5?

what is the probability I guess 3 or more right when I guess on 5 questions.
let P be the probability of a right answer. [1/2]
let Q be the probability of a false answer.[1/2]
[P+Q]^5=1
[P^5+5P^4Q+10P^3Q^2+10P^2Q^3+5PQ^4+Q^5=1

at least 3 right = all 5 correct OR 4 correct OR 3 correct
at least 3 correct= P^5+5P^4Q+10P^3Q^2
atleast 3 correct = [1/2]^5 + 5[1/2]^5+10/[1/2]^5
at least 3 correct= 16/32
at least 3 correct = 1/2 answer

Arthur