nh8404052006 wrote:There are 10 white marbels and 8 red marbles in a container. Mary is drawing out one marble at a time, without replacing it. What is the probability of drawing a white marble at N-th time?

The answer would be -
[8*..*(8-n+2)*10]/[18*17*..*(18-n+1)]

what is OA?

OA :[ 10C1*( 10+8-1P10+8-1 ) ]/ 10+8P10+8
= 10(/10+8)

Frankly, I do not understand the OA

nh8404052006 wrote:OA :[ 10C1*( 10+8-1P10+8-1 ) ]/ 10+8P10+8
= 10(/10+8)

Frankly, I do not understand the OA

In question you are asking about the "nth time" but in your answer I am not seeing variable "n". I am not clear with this answer. Please explain little more..

OA = Original Answer... I was also not aware of this but on this forum people use so do I.. nothing great.. All i mean is Answer from where this question if picked...

Sachindh,

No matter how I look at the OA, the OA does not make sense to me. I suspect the OA is wrong.
The source of the question : Probability_Note.doc

nh8404052006 wrote:There are 10 white marbels and 8 red marbles in a container. Mary is drawing out one marble at a time, without replacing it. What is the probability of drawing a white marble at N-th time?

This question is WAY beyond the scope of the GMAT.

The formula would be insanely complex, since you'd have to account for every previous possibility. For example, if we want to know the probability of drawing a while marble on the 5th draw, we'd have to add up the probability of every stream leading to a white one 5th, i.e.:

RRRRW
RRRWW
RRWWW
RWWWW
WWWWW
WRRRW
WRRWW

and so on and so on. We can use combinations and permutations to account for some duplicates (e.g. WRWRW has the same probability as WWRRW), but again the formula is going to be unbelievably complicated.

So, if you're applying for a Masters in Combinatorics, worry about this question. If you're studying for the GMAT, completely ignore it and focus on far simpler problems that may actually appear on the test. I'd also ignore every other question from this source, since the ones you've posted are generally far more difficult than anything on the GMAT.

nh8404052006 wrote:OA :[ 10C1*( 10+8-1P10+8-1 ) ]/ 10+8P10+8
= 10(/10+8)

Frankly, I do not understand the OA

By the way, the answer you posted (if accurate, you seem to have some operators in weird locations) is simply the chance of the first marble being white, which we can answer much more quickly using either common sense or the probability formula:

Prob = # desired outcomes/total # of possibilities = 10/18