Two fair dice are thrown. What is the probability that the absolute difference between the two numbers is 2 or 3?

Difficulty: Easy

Correct Answer: 7/18

Explanation:


Introduction / Context:
We count ordered pairs (a,b) with a,b ∈ {1,…,6} such that |a − b| ∈ {2,3}. Counting favorable outcomes directly is efficient for small dice problems.


Given Data / Assumptions:

  • Sample space size = 36 equally likely ordered pairs.
  • We need |a − b| = 2 or 3.


Concept / Approach:
Enumerate the pairs for |a − b| = 2 and for |a − b| = 3. Sum them, divide by 36.


Step-by-Step Solution:

|a − b| = 2 pairs: (1,3),(3,1),(2,4),(4,2),(3,5),(5,3),(4,6),(6,4) ⇒ 8.|a − b| = 3 pairs: (1,4),(4,1),(2,5),(5,2),(3,6),(6,3) ⇒ 6.Total favorable = 8 + 6 = 14; probability = 14/36 = 7/18.


Verification / Alternative check:
Symmetry around the main diagonal implies for each a there are at most two b values satisfying |a − b| = k; tabulation matches the counts above.


Why Other Options Are Wrong:
5/18 corresponds to only one of the difference values; 1/2 greatly overestimates the event; 3/11 is not tied to a 36-sized space.


Common Pitfalls:
Forgetting that outcomes are ordered, or double-counting/omitting symmetric pairs.


Final Answer:
7/18

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