A certain bag of gemstones is composed of two-thirds

Question

A certain bag of gemstones is composed of two-thirds diamonds and one-third rubies. If the probability of randomly selecting two diamonds from the bag, without replacement, is 5/12, what is the probability of selecting two rubies from the bag, without replacement?

(A) 5/36
(B) 5/24
(C) 1/12
(D) 1/6
(E) 1/4

(A) 5/36 
(B) 5/24 
(C) 1/12 
(D) 1/6 
(E) 1/4

Zarrolou · Accepted Answer

Diamonds=2x
Rubees=x

Two D=  \frac{2x}{3x}*\frac{(2x-1)}{(3x-1)}=\frac{5}{12} , solve for x and you get x=0(non sense) or x=3 (better).

Diamonds=2*3
Rubees=3

Probability of getting 2 rubees= \frac{3}{9}*\frac{2}{8}=\frac{1}{12}

KarishmaB · Answer

Say, a bag has 6 diamonds and 3 rubies. What is the probability of selecting 2 diamonds one after the other without replacement?

Probability of selecting one diamond = 6/9
Probability of selecting yet another diamond after selecting one = 5/8 (no of diamonds has gone down by 1 and total no. of diamonds has gone down by 1 too)
Total probability = (6/9)*(5/8)

Here, we assume that no of rubies is R and no of diamonds is 2R (since no of diamonds is twice the no of rubies)
Probability of selecting two diamonds without replacement = (2R/3R) * (2R - 1)/(3R - 1) = 5/12
Either cross multiply to get the value of R or try to plug in some values to see where you get a multiple of 12 in the denominator. 
Once you get the value of R as 3, you know the number of diamonds is 6.

Probability of picking two rubies one after the other without replacement = (3/9) *(2/8) = 1/12

guptaraja · Answer

is answer 1/12 

2/3 * (2X-1) / (3X-1) = 5/ 12 => X = 3 
So total gems = 9 
and probability of ruby = 1/3 * 2/8 = 1/12

Professor · Answer

this is good enough.........

    = 5/12
x^2-9x = 0
x = 0, 9

so x = 9
diamond = 6
ruby = 3
the prob (2 ruby) = 3c2/9c2 = 1/12

C.

Rock750 · Answer

Let R be the numbers of rubies in the bag,
we told that the selection is made without replacement in both cases ( selecting two diamonds or selecting two rubies)

Hence, we have :  \frac{2}{3}*\frac{2R-1}{3R-1}=\frac{5}{12}

So, the number of diamonds in the bag is 6. Likewise, the number of rubies in the bag is 3 and the total of the gemstones is 9.

The probability of selecting two rubies from the bag without replacement is :

\frac{1}{3}*\frac{2}{8}=\frac{1}{12}

Answer : C

nave · Answer

Looks like you got your diamonds and rubies mixed up :wink:

though you got it right later

Rock750 · Answer

u are right navy81, thx

Hope this silly mistake had not confused anyone. Anyway, i think it's OK by now :-D

jlgdr · Answer

(d/d+r)(d-1/d+r-1) = 5/12

d = 2r

Therefore r = 3
d= 6

Probability of 2 rubies is

(3/9)(2/8) = 1/12

C it is

b00gigi · Answer

Can someone explain the

2/3 * (2R-1)/(3R-1)

part?

EMPOWERgmatRichC · Answer

Hi All,

We can solve this problem by TESTing VALUES. However, we have so much specific information, we CANNOT TEST random values. We have to use the information in the prompt to pick a logical number that matches all of the given "restrictions"

Here's what we have to work with:
1) Since the gems can be broken down into 2/3 diamonds and 1/3 rubies, the TOTAL must be a MULTIPLE of 3.
2) Since the probability of pulling 2 diamonds is 5/12, when we multiply the two individual probabilities, we MUST end with a denominator that is a multiple of 12 (so the fraction can be reduced to 5/12).

Let's start at "3" and work up....

If there are 3 gems, then we have 2 diamonds.
The probability of pulling 2 diamonds is (2/3)(1/2) = 2/6 which is NOT a match.

If there are 6 gems, then we have 4 diamonds.
The probability of pulling 2 diamonds is (4/6)(3/5) = 12/30.....30 cannot reduce to 12. This is NOT a match

If there are 9 gems, then we have 6 diamonds.
The probability of pulling 2 diamonds is (6/9)(5/8) = 5/12...This IS a MATCH

So we have....
Total= 9 
Diamonds = 6
Rubies = 3

The question asks for the probability of selecting 2 rubies....

The probability of selecting the first ruby = (3/9)
The probability of selecting the second ruby = (2/8)
(3/9)(2/8) = 6/72 = 1/12

Final Answer:  C

GMAT assassins aren't born, they're made,
Rich

anairamitch1804 · Answer

The simplest way to solve the problem is to recognize that the total number of gems in the bag must be a multiple of 3, since we have 2/3 diamonds and 1/3 rubies. If we had a total number that was not divisible by 3, we would not be able to divide the stones into thirds. Given this fact, we can test some multiples of 3 to see whether any fit the description in the question. 
The smallest number of gems we could have is 6: 4 diamonds and 2 rubies (since we need at least 2 rubies). Is the probability of selecting two of these diamonds equal to 5/12? 
4/6 × 3/5 = 12/30 = 2/5. Since this does not equal 5/12, this cannot be the total number of gems. 
The next multiple of 3 is 9, which yields 6 diamonds and 3 rubies: 
6/9 × 5/8 = 30/72 = 5/12. Since this matches the probability in the question, we know we have 6 diamonds and 3 rubies. Now we can figure out the probability of selecting two rubies: 
3/9 × 2/8 = 6/72 = 1/12
The correct answer is C.

ScottTargetTestPrep · Answer

We are given that the bag contains two-thirds diamonds and one-third rubies. We are also given that the probability of randomly selecting two diamonds from the bag, without replacement, is 5/12.

Let’s represent the number of rubies in the bag by x. Since the number of diamonds is twice the number of rubies, the number of diamonds in the bag will be 2x.

The probability of choosing two diamonds from the bag, without replacement, can be represented in terms of x by (2x/3x)((2x - 1)/(3x - 1)).

We are also given that this probability is equal to 5/12. Thus:

(2x/3x)((2x - 1)/(3x - 1)) = 5/12

(4x^2 - 2x)/(9x^2 - 3x) = 5/12

Cross multiply to obtain:

48x^2 - 24x = 45x^2 - 15x

3x^2 - 9x = 0

3x(x - 3) = 0

From this equation, we obtain x = 0 or x = 3. Since we know the bag is not empty, x must equal 3, and thus there are 6 diamonds and 3 rubies in the bag. Now the probability of choosing two rubies from the bag, without replacement, can be calculated to be (3/9)(2/8) = (1/3)(1/4) = 1/12.

Alternate Solution:

We can let T = the total number of gems, (2/3)T = diamonds, and (1/3)T = rubies.

Since the probability of randomly selecting two diamonds from the bag, without replacement, is 5/12, we can create the following equation:

x   = 5/12

(2/3) x ((2/3)T - 1)/(T - 1) = 5/12

((2/3)T - 1)/(T - 1) = 5/8

Cross multiply, we have:

(16/3)T - 8 = 5T - 5

16T - 24 = 15T - 15

T = 9

We know that there are twice as many diamonds as rubies. Thus, there are 6 diamonds and 3 rubies, so the probability of selecting two rubies from the bag is:

3/9 x 2/8 = 1/3 x 1/4 = 1/12

Answer: C

Vinayak1996 · Answer

Hope this helps
Probabiltity of selecting 2 diamonds we GET
((2/3)x/x) × ((2/3)x-1)/(x-1)=5/12

On solving the above equation we get x=9
Number of rubies=1/3 of 9 is 3
Therefore probability of secleting 2 rubies from 9 gemstones is given by

3C2/9C2=1/12

Hence C

Posted from my mobile device

DeeptiManyaExpert · Answer

Take number of diamonds as 2x, so rubies will be x and total number if gemstones will be 3x.

(2x/3x) *  Answer - C

PS: You can not plug in a number for diamonds and rubies in this question.

CEdward · Answer

Hi guys, 
Just wondering where I went wrong. I did something similar.

d = diamonds

(d/12) x (d - 1)/11 = 5/12

Couldn't solve for d via quadratic though...

MBAB123 · Answer

CEdward , you're essentially assuming the total gemstones to be 12, which would automatically give you the total diamond to be 8 and lead you to the wrong answer.

gmatmba12 · Answer

Question on this:

If I set probability of diamonds, I get

2/3 X   = 5/12

y = 5/8 (in other words the probability of the second being a diamond is 5/8.

We know the probability of rubies would be 1/3 X P(second being ruby)

Why can't I subtract 5/8 from 1 since that would be the probability of rubies and do 1/3 X 3/8 = 1/8?

KarishmaB · Answer

5/8 is the probability of selecting diamond the second time AFTER selecting a diamond first. 
1 - 5/8 will give you the probability of selecting a ruby the second time AFTER selecting a diamond first.

Whereas when you do 1/3 X 3/8, you are assuming that you selected a ruby first and then a ruby the second time around after selecting a diamond first. Hope you see that this doesn't make sense. You cannot use 3/8 here.

Check in my solution above. The probability of selecting a ruby second time after selecting a ruby first time is 2/8.
Note that 5/8 and 2/8 do not add up to 1 because they are dependent on different first events.

Natansha · Answer

Hi Karishma, can we cross R in 2R/3R since R can't be 0 or negative?

Also since diamonds are two times as rubies, why can't the probability of ruby be 1/2 that of diamond?

A certain bag of gemstones is composed of two-thirds

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