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njvenkatesh
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GMAT probability 22 Jul 2006, 07:18
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Can someone help explaining this question ?

Thanks in advance



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vij101
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GMAT probability 22 Jul 2006, 08:47
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njvenkatesh
Can someone help explaining this question ?

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Probablility = Favorable events /total events.
Probability of selecting 3 apples from 10 = 10C3= 120.
Probability that two will be red and one will be green is 7C3*3C1 = 63

7C3*3C1/10C3 = 21/40
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njvenkatesh
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GMAT probability 22 Jul 2006, 11:04
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Thanks,

Could you please explain How you have arrived "7C3*3C1" ?
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pravinkawadkar
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GMAT probability 22 Jul 2006, 11:52
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You can select 3 red balls from 7 red balls in 7C3 ways and 1 green from 3 gree balls by 3c1 ways. So one can select 3 balls as 7C3*3c1 ways. If your question is why to multiply. Just imagine problem like this there are 4 ways from mumbai to singapore and 3 ways from singapore to Tokyo. So for each way from Mumbai to singapore you have 3 ways from singapore to tokyo so you have total 4*3 = 12 ways from mumbai. On same line for any two red balls combination you can select one green ball. There are such 21 red ball possible combination while 3 green ball possible combination. You can select gren ball in three way so toatl is 21*3 =63.
If you are struggling with probability and permutations then I will sugest you should read an ebook on probability from https://www.4gmat.com. Short but one reading is sufficent enough to boost confidence in this area. They have litlte tough prob but if one just understands is sufficent enough. It si little unknown book.
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jeunesis
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GMAT probability 23 Jul 2006, 12:59
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Futuristic
Its 7C2 X 3C1, IMO
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yup it should be...
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jaynayak
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GMAT probability 26 Jul 2006, 03:01
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The possible ways 2 red and 1 green apple can be chosen

1) RRG
2) RGR
3) GRR

Probability of 1) = 7/10 * 6/9 * 3/8 = 7/40
Probability of 2) = 7/10 * 3/9 * 6/8 = 7/40
Probability of 3) = 3/10 * 7/9 * 6/8 = 7/40

total = 7/40 + 7/40 + 7/40 = 21/40



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Hi there,
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