# Homework Help: Expected value

1. Mar 28, 2013

### ParisSpart

A question on a multiple choice test has 6 answers which just one is right and the other wrong.

If the amount of the correct answer is 10 points how many points should be substracted for a wrong answer so if nobody answers randomly choosing one of the 6 answers get, on average, 0 points?

I am trying to find the expected value like this : E(X)=10*P(X=10)+0*P(X=0) but the quiz doesnt take it correct where i am wrong?

2. Mar 28, 2013

### Staff: Mentor

That is the expected value if wrong answers do not give negative points.
The answer to the problem is not an expectation value, but the amount of negative points the wrong answers should get.

3. Mar 28, 2013

### ParisSpart

and how i can fo this?

4. Mar 28, 2013

### Staff: Mentor

What is "fo"?

Use some variable for the amount of negative points, and modify your formula for the expectation value accordingly. What do you know about this expectation value (given in the problem statement)?

5. Mar 28, 2013

### ParisSpart

i will not use expected value?

6. Mar 28, 2013

According to the Texas Department of Education Trilingual English/Spanish/Ebonics Program, "two 'n' dos be fo".

7. Mar 28, 2013

### Bacle2

You want the expected number of points for a response to be zero:

If you answer correctly , you get 10 points, like you said. If ( I'm assumming) you are

answering randomly, what is your probability of guessing correctly? What is your

probability of guessing incorrectly , if you guess at random?

You have the right set up, just follow-up on it.

8. Mar 28, 2013

### Staff: Mentor

This is unacceptable for homework, you've done this repeatedly. Going forward, you must use the homework template or your posts will be deleted.

9. Mar 28, 2013

### SammyS

Staff Emeritus
LOL !

I think that be f0 .