x Turn on thread page Beta
 You are Here: Home >< Maths

# Proabability question watch

1. A bag has 4 counters. 2 are worth £2, 1 is worth £4 and 1 is worth £8.
(a) You randomly select a counter, gaining £X. Find E(X) and Var(X)
(b) You place the first counter in your pocket. You then draw a second
counter from the bag at random. This counter has value £Y . Without further calculations,
write down Exp(y) and var(y)

I am not quite sure how to do b, would you have to consider each case seperatly??
2. (Original post by number23)
I am not quite sure how to do b, would you have to consider each case seperatly??
No, no calculation is required.

This may help.

Imagine you just take a first counter out of the bag, and without looking at it put it in your pocket.

Then take the second counter. What's the expectation/variance of this second selection.

I can think of one more scenario that might give a clue, if you're still stuck.
3. (Original post by ghostwalker)
No, no calculation is required.

This may help.

Imagine you just take a first counter out of the bag, and without looking at it put it in your pocket.

Then take the second counter. What's the expectation/variance of this second selection.

I can think of one more scenario that might give a clue, if you're still stuck.
I think the expectation and variance are the same but not sure exactly why, what was ur other example?
4. (Original post by number23)
I think the expectation and variance are the same but not sure exactly why, what was ur other example?
Yes they are.

Put your hand in the bag, pick a coin, but don't pull it out. Let it go, and pick a different coin and pull that one out.
5. (Original post by number23)
I think the expectation and variance are the same but not sure exactly why, what was ur other example?
If you are expected to pick a £4 counter, and there is a £4 counter in the bag, what would be the effect if you took it out?

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 11, 2013
Today on TSR

### Been caught plagiarising...

...for the 2nd time this year

### Mum says she'll curse me if I don't go to uni

Discussions on TSR

• Latest
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE