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Fix the postion of two brothers. This leaves us with 18 people.
So there 18 ways in which a person can sit between two brother. Now swap thsoe bothers we get another 18 ways
So combinations = 2 * 18.
Consider the groupe of three people a single entity
we have another 17 people left so there are 18 entities to be arranged
Around a circle. Tis can be done in (18-1)! = 17!