A bag consists of 20 marbles, of which 5 are blue, 9 are red

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BTGmoderatorLU: Moderator; Posts: 2505; Joined: Sun Oct 15, 2017 1:50 pm; Followed by:6 members

A bag consists of 20 marbles, of which 5 are blue, 9 are red

by BTGmoderatorLU » Tue Apr 10, 2018 3:22 pm

A bag consists of 20 marbles, of which 5 are blue, 9 are red, and the remainder are white. If Lisa is to select a marble from the bag at random, what is the probability that the marble will be red or white?

A. 3/4
B. 1/2
C. 1/4
D. 1/8
E. 1/16

The OA is A.

So, white = 20 - 5 - 9 = 6.

The probability that the marble will be red or white = probability that the marble will be red + probability that the marble will be white.

Probability that the marble will be red or white = 9/20+6/20 = 15/20 = 3/4.

Has anyone another suggestion about how to solve this PS question? Thanks in advance.

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Source: Beat The GMAT — Problem Solving | Tue Apr 10, 2018 3:22 pm

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Jay@ManhattanReview: GMAT Instructor; Posts: 3008; Joined: Mon Aug 22, 2016 6:19 am; Location: Grand Central / New York; Thanked: 470 times; Followed by:34 members

by Jay@ManhattanReview » Wed Apr 11, 2018 5:46 am

BTGmoderatorLU wrote:A bag consists of 20 marbles, of which 5 are blue, 9 are red, and the remainder are white. If Lisa is to select a marble from the bag at random, what is the probability that the marble will be red or white?

A. 3/4
B. 1/2
C. 1/4
D. 1/8
E. 1/16

The OA is A.

So, white = 20 - 5 - 9 = 6.

The probability that the marble will be red or white = probability that the marble will be red + probability that the marble will be white.

Probability that the marble will be red or white = 9/20+6/20 = 15/20 = 3/4.

Has anyone another suggestion about how to solve this PS question? Thanks in advance.

All's fine with this.

-Jay
_________________
Manhattan Review GMAT Prep

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[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pm; Location: Palo Alto, CA; Thanked: 2867 times; Followed by:511 members; GMAT Score:800

by [email protected] » Wed Apr 11, 2018 4:54 pm

Hi All,

We're told that a bag consists of 20 marbles: 5 are blue, 9 are red, and the remainder are white. We're asked for the probability of randomly selecting one marble that will EITHER be red OR white.

When it comes to probability questions, sometimes it's easiest to determine the probability of what we DON'T want - and then subtract that fraction from the number 1 to determine the probability of what we DO want.

Here, the only outcome that does NOT match what we're looking for is when we pull a blue marble. Since 5/20 of the marbles are blue, there is a 1/4 chance of pulling a marble that we DON'T want. Thus, there is a 1 - 1/4 = 3/4 probability of pulling a marble that we DO want (red or white).

Final Answer: A

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

Contact Rich at [email protected]

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8086; Joined: Sat Apr 25, 2015 10:56 am; Location: Los Angeles, CA; Thanked: 43 times; Followed by:29 members

by Scott@TargetTestPrep » Thu Jul 05, 2018 3:52 pm

BTGmoderatorLU wrote:A bag consists of 20 marbles, of which 5 are blue, 9 are red, and the remainder are white. If Lisa is to select a marble from the bag at random, what is the probability that the marble will be red or white?

A. 3/4
B. 1/2
C. 1/4
D. 1/8
E. 1/16

We see that there are 20 - 5 - 9 = 6 white marbles in the bag. Thus, the probability of selecting a red marble OR a white marble is 9/20 + 6/20 = 15/20 = 3/4.

(Note: We add the probabilities because we are interested in the probability, in one draw, that either of the two events will happen.)

Answer: A

Scott Woodbury-Stewart
Founder and CEO
[email protected]