Math Is Fun Forum

ziggs

Member

Registered: 2009-02-22

Posts: 7

Uniform Distribution / Conditional Probability

So I just took a probability test and I'm having a hard time with the fact that my answer is wrong. I've done some research online and I believe I am correct, I was hoping to get some input. I'm new to using LaTeX so sorry if it's sloppy. Thanks!

Problem: Suppose that the amount of time in minutes that I wait for a bus is uniformly distributed on the interval [0,60].

(I got part A correct)
Part A: What is the probability I have to wait for at least 20 minutes?

Solution:

(I got part B wrong)
Part B: Suppose that when I arrive at the bus stop, I meet someone else who has already been waiting for the same bus for 20 minutes. If I take that information into account, what is the probability that I will have to wait for at least 20 minutes?

Solution:

Since someone else has already been waiting for the same bus for 20 minutes and has not been picked up yet, I would think we could change the sample space? So I used an interval of length 40, since the original was 60.

ziggs

Member

Registered: 2009-02-22

Posts: 7

Re: Uniform Distribution / Conditional Probability

P.S. I used this site:

http://cnx.org/content/m16819/latest/

Example 2, Problem 3.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Uniform Distribution / Conditional Probability

I can't find that problem on the page you linked, but I agree with your answer. Given that no bus has come in 20 minutes, the waiting time has to be somewhere in the interval [0,40].

---

Why did the vector cross the road?
It wanted to be normal.