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Official Solution:

A deck of cards contains 6 cards numbered from 1, 2, 3, 4, 5, and to 6. If three cards are randomly selected one by one from the deck without replacement, what is the probability that the numbers on the cards are consecutive integers in increasing order?

A. \(\frac{1}{60}\)
B. \(\frac{1}{30}\)
C. \(\frac{1}{20}\)
D. \(\frac{1}{6}\)
E. \(\frac{1}{3}\)


The number of ways to choose 3 numbers out of 6 when order matters is \(P^3_6=\frac{6!}{3!}=6*5*4=120\);

The number of ways to get 3 consecutive numbers in increasing order is 4: {1, 2, 3}, {2, 3, 4}, {3, 4, 5} and {4, 5, 6};

The probability is therefore, \(\frac{4}{120}=\frac{1}{30}\).


Answer: Bhanks for your explanation.
I want to know, why in this case order matter and we need to calculate total ways to choose 3 cards using Permutation instead of a regular combinatory

When selecting 3 cards, they can be chosen in various orders, such as {4, 3, 5} or {3, 1, 6} or {5, 3, 2} or ... Only a few of these will be in increasing order of consecutive integers, like {1, 2, 3}, {2, 3, 4}, {3, 4, 5}, and {4, 5, 6}. Therefore, we use 6P3 to find all the possible ways to select 3 cards, and then calculate the probability based on how many of those are consecutive in increasing order.
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I like the solution - it’s helpful.
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