OG Marbles in Bags Q

AbeNeedsAnswers wrote:----------# of --------% blue
---------marbles
Bag P 37 10.8%
Bag Q X 66.7%
Bag R 32 50.0%

If 1/3 of the total number of marbles in the three bags listed in the table above are blue, how many marbles are there in bag Q?

A) 5
B) 9
C) 12
D) 23
E) 46

B

There are a total of 37 + X + 32 = 69 + X number of marbles; and out of them 1/3 of (69 + X) = (69 + X)/3 are blue.

Individually, the number of blue marbles in each bag is given by...

Bag P: 37 x 10.8%.

It' calculation is challenging without a calculator; however, we can get this.
We see that 10% of 37 = 3.7 and 11% of 37 = 3.7 + 0.37 = 4.07; also 10.8% lies between 10% and 11%, thus the value of 10.8% of 37 would lie between 3.7 and 4.07. Since the number of blue marbles is a positive integer and only one integer, 4, lies between 3.7 and 4.07, the avlue of 10.8% of 37 = 4.

Bag Q: X x 66.7% = X*(2/3) = 2X/3; 66.67% is a popular figure and one must remember that it is equal to 2/3.

Bag R : 32 x 50.0% = 32*1/2 = 16

Thus, we have,

4 + 2X/3 + 16 = (69 + X)/3
2X/3 + 20 = 69/3 + X/3

2X/3 – X/3 = 23 – 20

X/3 = 3

[spoiler]X = 9.[/spoiler]

The correct answer: B

Hope this helps!

Download free ebook: Manhattan Review GMAT Quantitative Question Bank Guide

-Jay
_________________
Manhattan Review GMAT Prep

Locations: New York | Bangkok | Abu Dhabi | Rome | and many more...

Schedule your free consultation with an experienced GMAT Prep Advisor! Click here.

Hi Abe Needs Answers,

This question involves a particular Number Property that you can use to eliminate Answers; combined with TESTing THE ANSWERS, you can solve this problem with just a bit of arithmetic.

From the table, we know that the total number of marbles is 37 + X + 32 = (69 + X) marbles. The prompt tells us that exactly 1/3 of the total marbles are blue, so (69+X) MUST be a multiple of 3.

69 is a multiple of 3; since we're adding X to 69 and ending up with a multiple of 3, then X must ALSO be a multiple of 3. Eliminate Answer A, D and E.

From here, we can TEST THE ANSWERS. Let's TEST...

Answer B: 9

IF... X = 9
Bag P = (about 10.8%) of 37 = 4 blue marbles
Bag Q = (about 66.7%) of 9 = 6 blue marbles
Bag R = (50%) of 32 = 16 blue marbles

Total Blues = 26
Total Marbles = 78
26/78 = 1/3. This is an exact match for what we were told, so this MUST be the answer.

Final Answer: B

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