bhushangiri wrote:

arjtryarjtry wrote:

There are 7 keys in a key ring. If two more keys are to be added in the ring at random, what is the probability that two keys will be adjacent?

(A) 1/8

(B) 1/6

(C) 1/5

(D) 1/4

(E) 1/3

1/4.

Total number of ways to arrange 9 keys in circular permutation = 8!

Consider the 2 keys in question as unit. So you have 8 units which can be arranged in 7! ways. The pair can be arranged in 2 ways. So total ways in which the pair always stays together = 2*7!

Prob = (2*7!) / 8! = 1/4

Nice explanation!

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