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Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her [#permalink]
11 May 2006, 20:21

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Question Stats:

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Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her pocket. If she withdraws a number of these coins at random, how many coins would she have to withdraw to ensure that she has at least a 50 percent chance of withdrawing at least one quarter?

Prof, probability is not my strong part of the math, but anyway I will try my best shot.

My answer is 9 coins.
She must withdraw 9 coins before she has at least 50 percent chance of withdrawing at least one quarter.
Solution: she has 6 quarters. So to have at least 50% chance she must have left with 12 coins (6 quarters and 6 any of the other).
Total 21 coins minus 9 coins left her with needed 12 coins.

Hallo Prof and M8,
Agree with the approach presented by M8 i think there is a worse scenario. She withdraws 5 quarters, then she will heed to withdraw 14 coins from dimes and nickels to make sure that the chance is at least 50% so in total 19 coins can be withdrawn.

Hallo Prof and M8, Agree with the approach presented by M8 i think there is a worse scenario. She withdraws 5 quarters, then she will heed to withdraw 14 coins from dimes and nickels to make sure that the chance is at least 50% so in total 19 coins can be withdrawn.

BG we both forgot about the possibility that she could withdraw all quarters before she left with 12 or with 2 coins (dime and nickel). What do you think?

Agree M8
but in this case she will have nothing to draw and the Q has no sense. Think that this type of Q asks about the worse possible scenario i. e. when the maximum numbers of attempts should be made in order to have the chance of 50/50.

Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her pocket. If she withdraws a number of these coins at random, how many coins would she have to withdraw to ensure that she has at least a 50 percent chance of withdrawing at least one quarter?

Worst chance is when she has to draw 16 coins to get a quarter. Best chance is when she has to draw 1 coin.

So Total # of outcomes = 1, 2, 3, ... 16 = 16
# of desired outcomes = 50% of total # of outcomes
which is 8.

So, she has to draw 8 coins to give her a 50% chance that she draws 1 quarter...

BTW Prof and M8, when are you guys taking the Gmat...
mine may 25 and im doing real bad in math..

I was going through my error logs from last month for DS and an interesting thing I noticed. All the DS problems that I got wrong, about 90% of them had E as answer where I had guessed C. Result of premature guessing...without reading the questions carefully...and you guys have been very helpful so ... THANKS!

BTW Prof and M8, when are you guys taking the Gmat... mine may 25 and im doing real bad in math..

I was going through my error logs from last month for DS and an interesting thing I noticed. All the DS problems that I got wrong, about 90% of them had E as answer where I had guessed C. Result of premature guessing...without reading the questions carefully...and you guys have been very helpful so ... THANKS!

Buddy you are lucky one, you have plenty of time till May 25.
My G-Day in on May 16.
But I'm the apprentice to Professor, he is real genius in Math.
My scaled score in Math is stable - 48. It's proved by real GMAT I passed last October and by a couple of GMATPreps. Trying to improve it in the course of about 2 months but nothing can do - neither more nor less than 48 - stable.
Hope that next Thursday it will be no less than 48 or even more.

BTW Prof and M8, when are you guys taking the Gmat... mine may 25 and im doing real bad in math..

I was going through my error logs from last month for DS and an interesting thing I noticed. All the DS problems that I got wrong, about 90% of them had E as answer where I had guessed C. Result of premature guessing...without reading the questions carefully...and you guys have been very helpful so ... THANKS!

Buddy you are lucky one, you have plenty of time till May 25. My G-Day in on May 16. But I'm the apprentice to Professor, he is real genius in Math. My scaled score in Math is stable - 48. It's proved by real GMAT I passed last October and by a couple of GMATPreps. Trying to improve it in the course of about 2 months but nothing can do - neither more nor less than 48 - stable. Hope that next Thursday it will be no less than 48 or even more.

thanx guys for your nice words.... however i am nothing in comparision with the real genius people like honghu, laxi. if we really want to learn something in math, we should look on thier posts and approaches...

they are great.....

Last edited by Professor on 13 May 2006, 16:09, edited 1 time in total.

I think it helps to invert the problem. So whats the probability that you don't have any quarters after N coin withdrawals.

That would be F(N)=15/21*14/20.. (N terms). The probability that you have at least one quarter is (1-F(N)).

F(1) = 15/21 F(2) = (15/21) (14/20) = 0.5 Since 1-F(2) = 0.5, the answer is 2.

old_dream_1976 wrote:

it is 2 [6c2 + (6c1 x 15c1)]/21C2 but the more important question is should this be a trial and error p(1), p(2) ........ approach ? Can anyone suggest a more direct method ?

i guess the above approaches both work. the answer should be 2 as well. remember the question says "at least 1 qt".

prob of getting "at least 1 qt" in drawing 1 qt = 6/21 = 2/7 i.e less than 50%. so not correct.

prob of getting "at least 1 qt" in 2 coins = [(prob of getting getting 1 qt and 1 dime or nickel) + (prob of getting getting 2 qts)]/(total ways of geeting 2 coins)
prob = [(6 x 15) + (6c2)]/21c2 = (90+15)/210 = 50%

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