A and B stand in a ring with 10 others.if arrangement of 12 persons is at random then probability that there are exactly 3 persons between a and B is: A)16/21 B)5/21 C)8/21 D)none

Dear Varsha ,

ans for the above ques is (D) none of these.

this is becuse
1st we have to select 3 out of 10 persons...then arrange them randomly....then arrange other 7 n at last arrange given 2.....................and divide them by total no of possible arrangements(for circular permutation it is(n-1)!)

ans= 10C3 * 3!*7!*2/11! = 2/11