20 marbles were pulled out of a bag of only white marbles...

BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

This topic has expert replies

Post new topic Post Reply

Previous Topic Next Topic

swerve: Legendary Member; Posts: 2499; Joined: Sun Oct 29, 2017 2:04 pm; Followed by:6 members

20 marbles were pulled out of a bag of only white marbles...

by swerve » Mon Mar 12, 2018 10:04 am

Timer

00:00

Your Answer

Global Stats

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.

Quote

Source: Beat The GMAT — Problem Solving | Mon Mar 12, 2018 10:04 am

DavidG@VeritasPrep: Legendary Member; Posts: 2663; Joined: Wed Jan 14, 2015 8:25 am; Location: Boston, MA; Thanked: 1153 times; Followed by:128 members; GMAT Score:770

by DavidG@VeritasPrep » Mon Mar 12, 2018 10:35 am

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

Quote

GMAT/MBA Expert

Scott@TargetTestPrep: GMAT Instructor; Posts: 8083; Joined: Sat Apr 25, 2015 10:56 am; Location: Los Angeles, CA; Thanked: 43 times; Followed by:29 members

by Scott@TargetTestPrep » Wed Mar 14, 2018 3:34 pm

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

Answer: D

Scott Woodbury-Stewart
Founder and CEO
[email protected]