probability of questions on a test

if you take a multiple choice test that consists of 10 questions, what is the probability that you will get a mark of atleast 50% if you know 4 answers, and guess at the other 6. Each question has 5 potential answers.

I am not sure how exactly to tackle this problem. I know you have 1/5 to get each questions right that you guess at.

Several Levels of Reply

Literal Reading of Your Heading: I think the probability of questions on a test is 100%. I suppose there might be statements or prompts, but mostly there are questions.

To your Problem Statement: If you know 4, your question can be reduced to the relatively simple question, "What is the probability of getting at least one (1) of six (6) correct, given that guessing is 20% effective?" This can be further reduced mathematically by calculating the probability of missing ALL the six questions and subtracting it from unity.

Probability(Missing All 6) = 0.8^6
Probability(At least one correct) = 1 - 0.8^6