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# S1 probability help! watch

1. A bag contains 12 red discs and 10 black discs.
Two discs are removed at random, without replacement.
Find, to 3 dp, the probability both discs are red.

2 Another bag contains 7 green discs and 13 blue discs.
Three discs are removed at random, without replacement.
Find, to 3 dp, the probability that exactly two are green.

3 A third bag contains 33 discs, all of which are either yellow or brown.
Two discs are removed at random, without replacement.
The probability that both discs are yellow is 1/8
Find the number of yellow discs that were in the bag at first.

I could do the first two, but struggling with the 3rd question.

I literally have no clue how to answer this so I can't really give any of my working. - Sorry!

Thanks for any help
2. (Original post by L1000)
A third bag contains 45 discs, all of which are either yellow or brown.
Two discs are removed at random, without replacement.
The probability that both discs are yellow is 118.
Find the number of yellow discs that were in the bag at first.

I literally have no clue how to answer this so I can't really give any of my working. - Sorry!

Thanks for any help
It seems as if some information is missing, if its the third bag what about the 1st and 2nd? Also the probability of something happening can't be more than 1, so 118 is an incorrect probability
3. (Original post by raiden95)
It seems as if some information is missing, if its the third bag what about the 1st and 2nd? Also the probability of something happening can't be more than 1, so 118 is an incorrect probability
Sorry! Didn't check before copy and pasted. I edited it in my post.
4. Let X be the number of yellow ones. What is the prob of choosing one? And then another? This will give you a quadratic to solve.
5. You are starting with 33, so let X be the number chosen first and X-1 be the next because one has been chosen and 32 to choose from. (x/33)(x-1/32) =1/8 ? I haven't tried this type of question but I think that's the method

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