I'm stuck on this question: there are 7 red counters in a bag, the rest are blue. 2 counters were taken at random from the bag. The probability that here will be one of each colour is 7/15. What is the total number of colours in the bag?
I have tried to form a probability tree and I came to the (wrong) conclusion, n being number of counters, that ((7/n) x ((n-7)/(n-1))) x (((n-7/n) x (7/(n-1))) = 7/15 - sorry I realise there are an unreal amount of brackets in this but I tried to make it as clear as possible... that kinda backfired lol
If anyone could put me on the right track I would appreciate it! Or if you're feeling kind DM me the solution![]()
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mundosinfin
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- 02-02-2018 22:26
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ghostwalker
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- 02-02-2018 23:02
(Original post by mundosinfin)
I'm stuck on this question: there are 7 red counters in a bag, the rest are blue. 2 counters were taken at random from the bag. The probability that here will be one of each colour is 7/15. What is the total number of colours in the bag?
I have tried to form a probability tree and I came to the (wrong) conclusion, n being number of counters, that ((7/n) x ((n-7)/(n-1))) x (((n-7/n) x (7/(n-1))) = 7/15 - sorry I realise there are an unreal amount of brackets in this but I tried to make it as clear as possible... that kinda backfired lol
If anyone could put me on the right track I would appreciate it! Or if you're feeling kind DM me the solution
I trust you understand why.
I get two possible values for n:
Spoiler:Show
10 or 21
Last edited by ghostwalker; 02-02-2018 at 23:04. -
mundosinfin
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- 03-02-2018 07:32
(Original post by ghostwalker)
Very close - just one small slip. Should be:
I trust you understand why.
I get two possible values for n:
Spoiler:Show
10 or 21
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