Answer:
the first function is correct
Answer:
The value is 
Step-by-step explanation:
From the question we are told that
The probability that the student knows the answer to the question is 
The probability that that the student will guess is 
The probability that that the student get the correct answer given that the student guessed is 
Here W denotes that the student gets the correct answer
Generally it a certain fact that if the student knows the answer he would get it correctly
So the probability the the student got answer given that he knows it is

Generally from Bayes theorem we can mathematically evaluate the probability that the student knows the answer given that he got it correctly as follows

=> 
=> 
Let the number be represented by x
2x=60+ 5x -20*2=60+5*-20
2x-5x=5x-5x+60 -40=60+-100
-3x=60 -40=-40
-3x/-3=60/-3
x=-20
Therefore the number is -20