Bag A contains red, white and blue marbles such that the red to white marble ratio is 1:3 and the white to blue marble ratio is 2:3. Bag B contains red and white marbles in the ratio of 1:4. Together, the two bags contain 30 white marbles. How many red marbles could be in bag A?
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MGMAT CAT - ratio'S prob
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- HSPA
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Bag A--- R : W = 2:6 and W : B = 6:9 so it is 2x:6x:9x = R:W:B
Bag B--- R : W = 1y:4y
given 6x+4y = 30 what is the value of 2x?
possible values of x and y: x=3,y=3 and y=6,x=1
Number of redballs = 2x = 6 or 2
Using options only 6 is there...
Bag B--- R : W = 1y:4y
given 6x+4y = 30 what is the value of 2x?
possible values of x and y: x=3,y=3 and y=6,x=1
Number of redballs = 2x = 6 or 2
Using options only 6 is there...
First take: 640 (50M, 27V) - RC needs 300% improvement
Second take: coming soon..
Regards,
HSPA.
Second take: coming soon..
Regards,
HSPA.
- Stuart@KaplanGMAT
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We've seen a good algebraic solution, so let's attack the question strategically, by backsolving.cuty wrote:Bag A contains red, white and blue marbles such that the red to white marble ratio is 1:3 and the white to blue marble ratio is 2:3. Bag B contains red and white marbles in the ratio of 1:4. Together, the two bags contain 30 white marbles. How many red marbles could be in bag A?
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When the answer choices are numbers and the question is simple (e.g. "how many red marbles could there be?"), then backsolving is often a very effective approach.
If you can't quickly eliminate some choices via logic/common sense, then start with either B or D. Let's try B.
If there are 3 red marbles in the first bag, then there must be 9 white marbles. If there are 9 white marbles, then there must be 13.5 blue marbles. Well heck, we can't have 1/2 a marble, so eliminate B. We saw that an odd value for red gave us a bad result, so let's get rid of A as well.
Next let's check D. If there are 6 red marbles in bag A, then there are 18 white marbles. 18 white marbles means 27 blue marbles, which is legal.
On to bag B! 18 white marbles in bag A means we have 12 white marbles in bag B (for our total of 30 white). Since 12 is a multiple of 4, we can satisfy our 1:4 ratio for bag B. Accordingly, 6 works - choose D!
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
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