Probability GMAT Q that I'm stuck with

[Guest]

In a certain board game, a stack of 48 cards, 8 of which represent shares in stock, are shuffled and then placed down. If the first 2 cards selected do not represent shares in stock, what is the probability that the third card selected will represent a share of stock?

I am thinking 40/48 X 39/47 X 8/46

But the answer is 4/23.


Any thoughts? Thanks.

[relentlesspursuito700plus]

The problem is not asking you what is the probability that you will first pick a non-stock card, then pick another non-stock, and then finally pick a stock-card. If you read carefully, it is saying, "Ok, you picked two cards and they were 'regular' cards. Now what is the probability that on your third pick you will get a stock-card?"

You did the right thing by reducing the denominator. However, you should not multiply the three possible out comes. I looked at each event independently. After the two non-stock cards are gone you are left with 46 cards as you correctly identified. This means you have 8/46 chance of picking a stock-card on your third try.

What you did shows the probability of getting exactly 1 stock-card on you first three attempts.

Stacey Koprince knows everything when it comes to GMAT.... can I get confirmation on my approach, Stacey?

I think MGMAT covers this in its Word Translation work book. If you haven't gotten MGMAT books, buy them all. And hrow away all the "mass-marketed" test prep books from "well-known" test prep companies. Go with MGMAT and supplement your practice with OG or some other independent tutors who can give you focused materials. If I hadn't already spend an insane amount of money with Vertias, I would actually take a MGMAT course in a heart beat just to work with Stacey.

Just a couple more weeks for me until D-Day!!

[Guest]

The problem is not asking you what is the probability that you will first pick a non-stock card, then pick another non-stock, and then finally pick a stock-card. If you read carefully, it is saying, "Ok, you picked two cards and they were 'regular' cards. Now what is the probability that on your third pick you will get a stock-card?"

You did the right thing by reducing the denominator. However, you should not multiply the three possible out comes. I looked at each event independently. After the two non-stock cards are gone you are left with 46 cards as you correctly identified. This means you have 8/46 chance of picking a stock-card on your third try.

What you did shows the probability of getting exactly 1 stock-card on you first three attempts.

Stacey Koprince knows everything when it comes to GMAT.... can I get confirmation on my approach, Stacey?

I think MGMAT covers this in its Word Translation work book. If you haven't gotten MGMAT books, buy them all. And hrow away all the "mass-marketed" test prep books from "well-known" test prep companies. Go with MGMAT and supplement your practice with OG or some other independent tutors who can give you focused materials. If I hadn't already spend an insane amount of money with Vertias, I would actually take a MGMAT course in a heart beat just to work with Stacey.

Just a couple more weeks for me until D-Day!!

Just to be SUPER clear. 8/46 chances, reduced, equals 4/23. Thanks.

[RonPurewal]

The problem is not asking you what is the probability that you will first pick a non-stock card, then pick another non-stock, and then finally pick a stock-card. If you read carefully, it is saying, "Ok, you picked two cards and they were 'regular' cards. Now what is the probability that on your third pick you will get a stock-card?"

You did the right thing by reducing the denominator. However, you should not multiply the three possible out comes. I looked at each event independently. After the two non-stock cards are gone you are left with 46 cards as you correctly identified. This means you have 8/46 chance of picking a stock-card on your third try.

What you did shows the probability of getting exactly 1 stock-card on you first three attempts.

Stacey Koprince knows everything when it comes to GMAT.... can I get confirmation on my approach, Stacey?

I think MGMAT covers this in its Word Translation work book. If you haven't gotten MGMAT books, buy them all. And hrow away all the "mass-marketed" test prep books from "well-known" test prep companies. Go with MGMAT and supplement your practice with OG or some other independent tutors who can give you focused materials. If I hadn't already spend an insane amount of money with Vertias, I would actually take a MGMAT course in a heart beat just to work with Stacey.

Just a couple more weeks for me until D-Day!!

Just to be SUPER clear. 8/46 chances, reduced, equals 4/23. Thanks.

this is correct. unfortunately, one of you must provide the source for this problem, so that we don't have to remove the thread (forum rules - see the sticky). it would be a shame to have to remove a thread that plugs our company in such a wonderful way, but forum rules are forum rules.

stacey is the realness.

[josesav7]

Hi Ron,
This was posted in one of the two CAT exams of GMAT Prep. Can you please explain why the answer would be 4/23?

I got this answer correct but I think I guessed it...

Thanks a bunch!

[messi10]

Once the two cards are selected, you are left with 46 cards. Out of those 46, 8 still represent shares in a stock.

Probability of drawing a stock card now is 8/46, i.e. the total desirable outcomes over total possible outcomes. The fraction can be further reduced to 4/23, which is the answer

[jnelson0612]

Once the two cards are selected, you are left with 46 cards. Out of those 46, 8 still represent shares in a stock.

Probability of drawing a stock card now is 8/46, i.e. the total desirable outcomes over total possible outcomes. The fraction can be further reduced to 4/23, which is the answer

Excellent response; this is a nice, simple way to conceptualize the problem.