Source: Official Guide for GMAT Review 2016 Data Sufficiency ; #7


Each of the marbles in a jar is

Each of the marbles in a jar is either red or white or blue. If one marble is to be selected at random from the jar, what is the probability that the marble will be blue?

1 Explanation


Stephen Mortensen

r + w + b = t, where r = number of red marbles, w = number of white marbles, b = number of blue marbles, and t = total marbles. In order to determine the probability of drawing a blue marble, we will need to determine the ratio of blue marbles to total marbles, or b/t.

In option (1), we learn that r = 8 and t = 24, so 8 + w + b = 24. We can't solve for b, however, nor can we find a ratio, because we still need more information on w.

(2) gives us the information that t/2 = w. We could write this as r + b = t - w, r + b = t - t/2, and r + b = t/2. This still doesn't give us enough information to find a proper ratio, however, because r, b, and t are still undetermined.

The combination of (1) and (2), though, does provide the answers we need. We know that r = 8 and t = 24, and that w = t/2 = 12. So, since we have all of our values except for b, we know that we have the necessary information to solve for b and find the probability, b/t.

If we want to carry it through and actually solve, 8 + 12 + b = 24, b = 24 - 20, b = 4, b/t = 4/24 = 1/6, or about 16.67% probability.

May 11, 2016 • Comment

Cydney Seigerman, Magoosh Tutor

Thanks for sharing your solution, Stephen! Great job working through this problem :)

May 12, 2016 • Reply

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Official Guide for GMAT Review 2016

Official Guide for the GMAT 2015 14th Ed.

Official Guide for the GMAT 13th Ed.

Official Guide for the GMAT 12th Ed.

Revised GRE PDF 2nd Ed.

Section 6.3 Data Sufficiency

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