statistic probability questions please helppppp?

Question

A multiple cjoice exam consists of questions that have 4 answer choice. A certain student has a probabilty of 0.8 of knowing the correct answer to any particular question. if he doesn't know that correct answer, he guesses at random from the available choices.
1.what is the probability that this student gets one particular question correct?
2.if this student gets a certain question correct, what is the probability that he really knew the answer to the question?

help~ i can't not figure out..

Answer 1

1:

p(correct) = 1*p(know) + (1/4) p(don't know)
= 1 * 0.8 + (1/4) * (1 - 0.8)
= 0.8 + 0.2/4
= 0.8 + 0.05
= 0.85

/***********************************/



2:
..uhh, look up "Bayes" on google
... but "back door" observe the result of
part#1

knowing part = 0.8 ... guessing part = 0.05

p(really knew) = 0.8/0.85 = 16/17



quoting here:
Probabilities and Bayes's Theorem
.

(1.1) Definition.
The probability of H conditional on E is defined as PE(H) = P(H & E)/P(E), provided that both terms of this ratio exist and P(E) > 0.[1]

Answer 2

(1). 0∙25 (or 25%).
Once the person guessed, the education factor does not come into play. So it's a one out of four chance.

(2). 1* 0∙8 = 0∙8 (per question).
Since the student is rated at a 0∙8 value, every correct answer will have a value of 0∙8.

If the student answered 100 questions and there were 100 questions on the paper, the about information on the student would suggest he/she would get a final mark of 85%. That is, 80 questions correct, and 5 question guessed.

Answer 3

Suppose 100 questions. He knows 80. From the other 20, he guesses 5 right and 15 wrong.

1. 85 correct answers out of 100 is 85%.
2. 80 answers he knew out of 85 correct is 94.12%.

Answer 4

pt. 1 - I say 85%
pt. 2 - It says the answer in your question. He knows it 80% of the time.

Even though he may be right 85% of the tme, only 80% of the time was he actually sure of the answer.


Or wait...
if he's right 85% of the time...\
and 80% of the was on him
the probability of him 'knowing his correct answers' is 80/85%
or %94.11


By the way, am I doing your homework for you ?

Answer 5

is it possible to answer this question without knowing the total number of questions in the exam?