In a particular year, a company awards scholarships to 4 applicants from a group of nn applicants. If the order of selection is not taken into account, the number of ways in which the 4 scholarship recipients can be chosen is 7315.

  1. (i)

    Find the number of ways in which the 4 scholarship recipients can be chosen if the order of selection is taken into account.

  2. (ii)

    Find the value of nn.

  3. In the year 2012, there are 13 scholarship applicants. The 13 applicants, which include the eventual 4 scholarship recipients Ann, Belle, Connie and Don, are required to attend a briefing. The applicants are to be seated in 2 rows as shown below:

    Find the number of ways in which the 13 applicants can be seated if all 4 scholarship recipients must sit together during the briefing.

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Worked solution

7(i)

1. Relate the number of combinations to the number of permutations and evaluate

Number of ways=7315×4!=175 560\text{Number of ways} = 7315 \times 4! = 175\,560

Answer

175,560

7(ii)

1. Formulate an equation for the number of ways to choose 4 recipients from $n$ applicants.

(n4)=7315\binom{n}{4} = 7315

2. Solve for $n$ by testing integer values or using a graphing calculator.

Estimating nn since n4≈7315×24=175560n^4 \approx 7315 \times 24 = 175560, we expect n≈1755604≈20.4n \approx \sqrt[4]{175560} \approx 20.4.
Testing integer values:
When n=21n = 21, (214)=5985\binom{21}{4} = 5985
When n=22n = 22, (224)=7315\binom{22}{4} = 7315

Hence, n=22n = 22.

Answer

n=22n = 22

7[2]

1. Find the number of ways to position a contiguous block of 4 seats within the two rows.

The 4 scholarship recipients must sit together, so they must occupy 4 consecutive seats in the same row.
Number of valid blocks of 4 seats in the top row (6 seats) =6−4+1=3= 6 - 4 + 1 = 3
Number of valid blocks of 4 seats in the bottom row (7 seats) =7−4+1=4= 7 - 4 + 1 = 4
Total valid positions for the block =3+4=7= 3 + 4 = 7

2. Formulate the total number of seating arrangements.

For each valid block position, the 4 scholarship recipients can be arranged among themselves in 4!4! ways.
The remaining 9 applicants can be arranged in the remaining 9 seats in 9!9! ways.
Total number of seating arrangements =7×4!×9!= 7 \times 4! \times 9!

3. Calculate the final value.

=7×24×362 880= 7 \times 24 \times 362\,880
=60 963 840= 60\,963\,840

Answer

60 963 84060\,963\,840

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