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100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote ## 20 marbles were pulled out of a bag of only white marbles... tagged by: swerve ##### This topic has 2 expert replies and 0 member replies ### Top Member ## 20 marbles were pulled out of a bag of only white marbles... 20 marbles were pulled out of a bag only white marbles, painted black, and then put back in. Then another 20 marbles were pulled out, of which 1 was black, after which they were all returned to the bag. If the percentage of black marbles pulled out the second time represents their percentage in the bag, how many marbles in total does the bag currently hold? A. 40 B. 200 C. 380 D. 400 E. 3200 The OA is D. Please, can any expert explain this PS question for me? I tried to solve it but I can't get the correct answer and I would like to know how can I form an equation for this problem. I need your help. Thanks. ### GMAT/MBA Expert Legendary Member Joined 14 Jan 2015 Posted: 2667 messages Followed by: 122 members Upvotes: 1153 GMAT Score: 770 swerve wrote: 20 marbles were pulled out of a bag only white marbles, painted black, and then put back in. Then another 20 marbles were pulled out, of which 1 was black, after which they were all returned to the bag. If the percentage of black marbles pulled out the second time represents their percentage in the bag, how many marbles in total does the bag currently hold? A. 40 B. 200 C. 380 D. 400 E. 3200 The OA is D. Please, can any expert explain this PS question for me? I tried to solve it but I can't get the correct answer and I would like to know how can I form an equation for this problem. I need your help. Thanks. We know that 20 marbles were painted black after the first selection. We know that 1 of the 20 marbles from the second selection was black. If this represents the proportion of black marbles in the whole bag, it means that 1/20 of the marbles in the bag are black. Well, if we have 20 black marbles and this represents 1/20 of the total marbles, then 20 = (1/20) *T, and T = 400. The answer is D _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

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### GMAT/MBA Expert

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swerve wrote:
20 marbles were pulled out of a bag only white marbles, painted black, and then put back in. Then another 20 marbles were pulled out, of which 1 was black, after which they were all returned to the bag. If the percentage of black marbles pulled out the second time represents their percentage in the bag, how many marbles in total does the bag currently hold?

A. 40
B. 200
C. 380
D. 400
E. 3200
We can let x = the number of (white) marbles originally in the bag and create the following equation:

(# of black marbles in the bag)/(total # of marbles) = (# of black marbles pulled out 2nd time)/(total # of marbles pulled out 2nd time)

20/x = 1/20

x = 400

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