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

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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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by Brent@GMATPrepNow » Wed Apr 11, 2018 4:48 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.
Your solution is perfect.
If you're looking for an alternate approach, one option is to use the complement:

P(red or white) = 1 - P(NOT red or white)
= 1 - P(blue)
= 1 - 5/20
= 15/20
= 3/4

Answer: A
Brent Hanneson - Creator of GMATPrepNow.com
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