Feb/Mar 2022 p52 q5
2811
A group of 12 people consists of 3 boys, 4 girls and 5 adults.
How many different arrangements are there in which the 3 boys stand together and an adult is at each end of the line?
Solution
Consider the 3 boys as a single unit or 'block'. This reduces the problem to arranging 10 units: 1 block of boys, 4 girls, and 5 adults.
Since an adult must be at each end, we choose 2 adults to be at the ends. The number of ways to choose 2 adults from 5 is given by:
\(^5P_2 = 5 \times 4 = 20\)
Now, arrange the remaining 8 units (1 block of boys, 4 girls, and 3 adults) in the middle:
\(8! = 40,320\)
Within the block of boys, the 3 boys can be arranged among themselves in:
\(3! = 6\)
Therefore, the total number of arrangements is:
\(8! \times 3! \times ^5P_2 = 40,320 \times 6 \times 20 = 4,838,400\)
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