little guidence needed

hey .. so our teacher gave us a question sheet.. covering everything we've done .. and i've left it till the last minute ......i know ..i know ..my newyears resolution was not to be lazy as well .. so here 's the bits i'm stuck on

1) ...discrete random variables .. the second part of the question .. evaluate E(1/x) .. i know how to do expected value but what does the 1/x mean ??..what do you do differetnly?/

2) two bags contain red balls and blue balls . bag A contains 3 red balls and 2 blue balls . bag B contains 2 red balls and 4 blue balls two fair coins are tossed and bag A is elected if two heads are obtained . otherwise bag B is selected A ball is then chosen at random from the selected bag .
a) find the probability that bag A is selected ...b) find the probability that the chosen ball is red .. c) given that the chosen ball is red find the probability the bag A was the selected bag .......... i understand it abit ... i'm mainly not sure about what method to use i.e like do you use fractions like very basic probability , combinations .. or what ??

3)A youth club has 120 members of whom 45 are girls and 75 are boys . the leader would like to change the evening on which the meet and he decides to seek the views of the members on this proposed change

a) state what is meant by a census in this context
b) on advantage and one disadvantage of using a sample compared with a census
c) he decides to use a stratified sample of 24 members . the 2 strata being the girls and boys who are members of the club determine the numbers of girls and boys in the sample.. honestly i can't remember anything on sampling i haven't done it this year .. help would be much appreciated !!! thank

2) two bags contain red balls and blue balls . bag A contains 3 red balls and 2 blue balls . bag B contains 2 red balls and 4 blue balls two fair coins are tossed and bag A is elected if two heads are obtained . otherwise bag B is selected A ball is then chosen at random from the selected bag .

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a) find the probability that bag A is selected

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What the probability of throwing 2 heads?. There are 4 cases: HH, HT, TH, TT

...b) find the probability that the chosen ball is red

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\(\displaystyle \underbrace{(1/4)(3/5)}_{\text{P(A and R)}}\)+\(\displaystyle \overbrace{(3/4)(2/6)}^{\text{P(B and R)}}\)

See where that comes from?.

.. c) given that the chosen ball is red find the probability the bag A was the selected bag

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Are you familiar with Baye's Theorem?. \(\displaystyle P(A|R)=\frac{P(A\text{and}R)}{P(R)}\)

or \(\displaystyle P(A|R)=\frac{P(A)P(R|A)}{P(A)P(R|A)+P(A')P(R|A')}\)

The first one is a little easier, huh?.

thank you soo much for repling .,... okay so you use conditional probability (that's what you use right so for a) would it just be 1/4 because it's one chance out of 4

That is the probability of choosing bag A.

ahhhh .. thanks (Y)

sorry about this but do you times p(A and r) together and then add to p(b and r) ??

You mean for part c?.

If so, think about what the problem is asking. The probability that the ball is from bag A given that it is red.

P(A|R). So, we need the probability it is from bag A AND red divided by the probability it is red.

The probability it is red:

Case 1: it is red and it came from bag A. The probability we choose bag A is 1/4 because of the coin flip.

Next, bag A has 5 balls. 3 of which are red. So, the probability we choose a red from the bag is 3/5.

So, the probability it is from bag A AND red is (1/4)(3/5)=3/20.

Now, divide that by the answer in part b. That is P(A|R)

See?.

i think i see it ..thanks well i gave it a go anyway using what you said .. for b) i got 0.4 and c) 0.16 .... is that completely wrong ??

probability is really becoming my weak point .. i understand it when it's explained but i always seem to get the method wrong is that make sense .:S... any chance i can help on one more i'll write the question down anyway... In a class of 20 pupils 10 are welsh 7 are english and 3 are scottish a sub-committee of 6 of three pupils is to be formed and it is to choose it's members at random from the 20 pupils
calculate the pprobability that the sub-committee contains a) no pupils b) 2 pupils of each nationality

maths_arghh234 said:

i think i see it ..thanks well i gave it a go anyway using what you said .. for b) i got 0.4 and c) 0.16 .... is that completely wrong ??

probability is really becoming my weak point .. i understand it when it's explained but i always seem to get the method wrong is that make sense .:S... any chance i can help on one more i'll write the question down anyway... In a class of 20 pupils 10 are welsh 7 are english and 3 are scottish a sub-committee of 6 of three pupils is to be formed and it is to choose it's members at random from the 20 pupils
calculate the pprobability that the sub-committee contains a) no pupils b) 2 pupils of each nationality

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Start a new thread with a new problem.

Anyway, please review the problem for correctness and start a new thread.

ahh okay thanks anyway