Probability

Expert replies

4meonly: Legendary Member; Posts: 891; Joined: Sat Aug 16, 2008 4:21 amFollowed by: 1 member; Thanked: 27 times; GMAT Score:660(

by 4meonly » Fri Nov 14, 2008 6:04 am

There are 20 red marbles, 20 blue marbles, 5 white marbles and 5 green marbles in a bag. If two marbles are to be selected at random, without replacement, what is the probability of getting two marbles with the same color.

[spoiler]OA 16/49[/spoiler]

Like this post

Source: Beat The GMAT — Problem Solving | Fri Nov 14, 2008 6:04 am

jimmiejaz: Master | Next Rank: 500 Posts; Posts: 207; Joined: Sun Mar 11, 2007 6:16 pm; Location: Mumbai; Thanked: 11 times

by jimmiejaz » Fri Nov 14, 2008 6:23 am

so, we want balls of the same color, which can be any one of the events:
2 red or 2 blue or 2 white or 2 green

In this case
the probability without replacement...
p = 20/50 * 19/49 + 20/50 * 19/49 + 5/50 * 4/49 + 5/50 * 4/49
Solve, we get probability = 16/49.
Hope it helps.

What if i have not yet beat the beast, I know i will beat it!!!!!!!!

Like this post

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Fri Nov 14, 2008 8:06 am

If u do it like u did yes we get 16/49. Agreed...

Lets say we did:

20C1*19C1+20C1*19C1+5C1*4C1+5C1*4C1 / 50C2 why do we end up getting 32/49 and not 16/49??

What am I missing here? Any ideas?

Like this post

logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

by logitech » Fri Nov 14, 2008 9:24 am

cramya wrote:If u do it like u did yes we get 16/49. Agreed...

Lets say we did:

20C1*19C1+20C1*19C1+5C1*4C1+5C1*4C1 / 50C2 why do we end up getting 32/49 and not 16/49??

What am I missing here? Any ideas?

50C2 = 50x49/2

it is a listing rather than comb. so 50C2 doubles your chances. and you have 32/49 instead of 16/49

LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"

Like this post

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Fri Nov 14, 2008 9:27 am

it is a listing rather than comb

Pl explain more on wat u exactly mean by this

Like this post

logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

by logitech » Fri Nov 14, 2008 9:52 am

cramya wrote:
it is a listing rather than comb
Pl explain more on wat u exactly mean by this

Cramya:

Lets say we have 3 marbles one red, one blue, and one yellow

what is the probability of picking 2 marbles, Red and Blue, without replacing them.

Well

1/3x1/2=1/6

In your solution

1c1 x 1c1/3c2 = 1/3

Because you are actually finding the probability of A marble/ 2 marbles

Going back to our original problem the probability you found is not:

Probability of X / SET X

BUT;

Probability of X / (CHOOSEN SET X )

Is it clear now ?

LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review