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Natalya put 14 blue marbles, 14 red marbles, and no other marbles into 3 empty cups. She put 2 blue...

GMAT Two Part Analysis : (TPA) Questions

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Two Part Analysis
Quant - Core
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Natalya put 14 blue marbles, 14 red marbles, and no other marbles into 3 empty cups. She put 2 blue marbles and 6 red marbles in Cup A, 6 blue marbles and 4 red marbles in Cup B, and 6 blue marbles and 4 red marbles in Cup C.

After this, Dmitry will randomly pick three marbles, one from each cup. Consistent with the given information, select for 3 blue the probability that Dmitry will pick 3 blue marbles, and select for 3 red the probability that Dmitry will pick 3 red marbles. Make only two selections, one in each column.

3 Blue
3 Red

0.090

0.120

0.125

0.150

0.250

0.500

Solution

Let's visualize this problem to make it crystal clear.

Phase 1: Owning the Dataset

Visualization

Let's create a simple table to organize our marble distribution:

Cup Blue Marbles Red Marbles Total
A 2 6 8
B 6 4 10
C 6 4 10
Total 14 14 28

We can verify our totals: \(2+6+6 = 14\) blue ✓ and \(6+4+4 = 14\) red ✓

Phase 2: Understanding the Question

Dmitry will randomly pick exactly one marble from each cup. We need to find:

  1. 3 Blue: The probability that all three marbles picked are blue
  2. 3 Red: The probability that all three marbles picked are red

Since he picks one marble from each cup independently, we'll multiply the individual probabilities.

Phase 3: Finding the Answer

Calculating P(3 Blue Marbles)

For Dmitry to pick 3 blue marbles, he needs:

  • A blue marble from Cup A: \(\mathrm{P} = \frac{2}{8} = \frac{1}{4}\)
  • A blue marble from Cup B: \(\mathrm{P} = \frac{6}{10} = \frac{3}{5}\)
  • A blue marble from Cup C: \(\mathrm{P} = \frac{6}{10} = \frac{3}{5}\)

Combined probability:
\(\mathrm{P(3\,blue)} = \frac{1}{4} \times \frac{3}{5} \times \frac{3}{5} = \frac{9}{100} = 0.090\)

Calculating P(3 Red Marbles)

For Dmitry to pick 3 red marbles, he needs:

  • A red marble from Cup A: \(\mathrm{P} = \frac{6}{8} = \frac{3}{4}\)
  • A red marble from Cup B: \(\mathrm{P} = \frac{4}{10} = \frac{2}{5}\)
  • A red marble from Cup C: \(\mathrm{P} = \frac{4}{10} = \frac{2}{5}\)

Combined probability:
\(\mathrm{P(3\,red)} = \frac{3}{4} \times \frac{2}{5} \times \frac{2}{5} = \frac{12}{100} = 0.120\)

Phase 4: Solution

Based on our calculations:

  • 3 Blue: 0.090
  • 3 Red: 0.120

These values match the answer choices provided, confirming our analysis.

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