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Sample space : allocate 6 seats out of 9 = 9C6 or 9C3 or 84

Arrange 6 people such that
No adjacent seats: 2 seats occupied in each row + where 1 row has one person at the middle

3C2*3C2*3C2 + 3C1*2C1*3C2 ( the row for 1 person --> select a row for 3 people --> arrange the two)

Adjacent seats = 1 - 45/84 = 13/28.
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In combinations problems like these. When one gets the required answer, IMO it is better to cross verify by scaling up the values to a common denominator and then compare if your answer was indeed right.
In this case we have 13/28, 14/28, 15/28, 16/28 and 18/28. So basically all the values are closeby and we need to be very careful in checking if we accidentally missed any single case or not that changes the answer.

In this case it is the fact that when the couple is having two choice [When all 6 occupy one of the two rows, essentially 3C2 possibilities]
The probability is about there existing couple of adjacent seats. So both those possibilities are essentially 1 case only for one type of case.


kevincan
A minibus has 3 rows of seats with 3 seats in each row. The six passengers who booked in advance have already taken their randomly assigned seats. A couple is next to board. What is the probability that there are two adjacent empty seats in the same row for the couple?

(A) \(\frac{13}{28}\)

(B) \(\frac{1}{2}\)

(C) \(\frac{15}{28}\)

(D) \(\frac{4}{7}\)

(E) \(\frac{9}{14}\)
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