When six unbiased dice are rolled simultaneously, the probability of getting all distinct numbers is
A. 1/6
B. 5/324
C. 1/54
D. 5/1296
GATE 2024 · Engineering Mathematics · Classical Probability and Counting · medium
Answer: P(all distinct) = 5/324 [Option B]
Count total outcomes: Each of 6 dice has 6 faces. Total = 6 x 6 x 6 x 6 x 6 x 6 = 6^6 = 46656.
Count favorable outcomes (all distinct): We need each die to show a different number from {1,...,6}. The first die can be any of 6 values, second any of 5 remaining, etc. Favorable = 6 x 5 x 4 x 3 x 2 x 1 = 6! = 720.
Compute probability and simplify: P = 720 / 46656. GCD(720, 46656) = 144. P = 5/324.