Eight students are shortlisted to be prize recipients for a competition. Two students are awarded the first prize and another two students are awarded the second prize. The eight shortlisted students are to stand in a single row for a photo-taking session with a guest of honour.

  1. Find the number of different possible arrangements if the two first prize awardees must stand together while the two second prize awardees also must stand together during the session.

  2. After the photo-taking session, the eight students proceed for a tea reception and are to be seated at a round table with 10 identical seats. If 4 students are to be seated on each side of the guest of honour at the round table, find the total number of possible seating arrangements.

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Worked solution

6[0]

1. Identify the number of distinct units to be arranged.

By grouping the two first prize awardees as 11 unit and the two second prize awardees as another 11 unit, the remaining 44 students act as individual units.
Total units to arrange =1+1+4=6= 1 + 1 + 4 = 6 units.

2. Calculate the total number of arrangements by multiplying the permutations of the units by the internal permutations of the groups.

Number of ways to arrange the 66 units =6!= 6!
Number of ways to arrange the first prize awardees within their group =2!= 2!
Number of ways to arrange the second prize awardees within their group =2!= 2!

Total possible arrangements =6!×2!×2!= 6! \times 2! \times 2!
=720×2×2= 720 \times 2 \times 2
=2880= 2880

Answer

2880

6[1]

1. Fix the guest of honour to resolve the circular permutation and identify the available seats.

Fix the guest of honour at one seat. To account for the circular arrangement, there is 11 way to do this since the 1010 seats are identical.
This leaves 99 remaining seats: 44 on the left, 44 on the right, and 11 directly opposite the guest of honour.

2. Determine the position of the empty seat and arrange the students.

Since exactly 44 students must be seated on each side (left and right) of the guest of honour, they must occupy all 44 seats on the left and all 44 seats on the right. The seat directly opposite must therefore be the single empty seat.
The 88 students can be arranged in these 88 specific seats in 8!8! ways.
8!=403208! = 40320

Answer

40320

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