A very challenging probability problem from 800 score.com

[kesho]

http://img25.imageshack.us/img25/842...00914724pm.png

[smbabu]

Dear Kesho,
It is really a great problem. is the answer 7?

Would you give me the referrence form where you have taken it.

Thanking you-
smbabu
sabyasachymistry@yahoo.com

[SuperFlanker]

I also concur with 7 because the worst case sequence is R G B R G B

[kesho]

Yes the ans is 7.

Please explain in details.

[MTAB1380]

Dear Kesho i believe that the way to get the answer is as follows:
No. of marbles = 18
No. of Red marbles = 8
No. of Green marbles = 4
No. of Black marbles = 6
Now,
The probability of picking up 3 red's = 1/R = 3/8
The probability of picking up 3 greens = 1/G = 3/4
The probability of picking up 3 blacks = 1/B = 3/6 or 1/2

Hence the probability of picking up 3 of the same colour is =

1/P = 1/R*1/G*1/B
= 3/8*3/4*1/2
= 9/64

Finally,
1/P = 9/64
P = 64/9
P = 7.111111 ~ 7
Hence, P = 7

Hope this answers the Q?

Please do correct me if i am wrong.

[kesho]

I also concur with 7 because the worst case sequence is R G B R G B

Understood.

@ MTAB1380, superflanker, smbabu

thanks

[smbabu]

Dear kesho,
Thank you very much. is 'MTAB 1380' available at internet?
-
smbabu