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GATE 2021 ME (ME2) – Question 7

General Aptitude · Quantitative Aptitude: Arithmetic and Number Computation · 2 marks · Multiple choice

A box contains 15 blue balls and 45 black balls. If 2 balls are selected randomly, without replacement, the probability of an outcome in which the first selected is a blue ball and the second selected is a black ball, is

  1. $3/16$
  2. $45/236$
  3. $1/4$
  4. $1/3$

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Show answer and explanation

Correct answer: (B) $45/236$

Explanation

First-blue probability is 15/60. Conditional on that event,45 black balls remain among 59 balls.

Multiply: $(15/60)(45/59)=\mathbf{45/236}$, **B**. Using 60 for the second denominator would assume replacement.