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
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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

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