A personality test assesses 32 key traits. These 32 traits are grouped into 4 domains - Cognition, Influence, Rapport and Planning - with each domain consisting of 8 traits. The test result for a person consists of his/her top 5 traits in ascending order of strength.
- (a)
Find the number of different results that the personality test can produce.
- (b)
Find the number of different results that can be produced with traits from at least 2 domains.
- (c)
Find the number of different results that can be produced with at least one trait from each domain and neither of the top 2 traits are from the Cognition domain.
- (d)
A result contains 2 particular traits from Rapport, 2 particular traits from Planning and 1 particular trait from Influence. Find the number of different possible results that can be produced with no consecutive traits from the same domain.
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Worked solution
9(a)
1. Identify that the result is an ordered selection, so permutations are required.
The test result consists of 5 distinct traits selected from 32.
Since the 5 selected traits are presented in ascending order of their specific strengths for that person, the sequence of the traits matters (i.e. changing the order of the same 5 traits produces a different result). Therefore, this is a permutation problem.
2. Calculate the total number of permutations.
9(b)
1. Identify the total number of unrestricted results (from part a).
Since a result consists of 5 traits arranged in ascending order of strength, order matters.
2. Calculate the number of results where all 5 traits belong to exactly 1 domain.
There are 4 domains, each with 8 traits. For all 5 traits to be from the same domain, we choose 1 domain, then arrange 5 of its 8 traits.
3. Use the complement method to find the number of results with traits from at least 2 domains.
9(c)
1. Determine the required distribution of traits among the 4 domains.
Since the result consists of traits and must include at least one trait from each of the domains, exactly one domain must provide traits, while the other three domains provide trait each.
The valid domain distributions can be divided into two cases:
- Case 1: The Cognition domain provides traits.
- Case 2: One of the other domains provides traits.
2. Calculate the number of valid results for Case 1 (Cognition provides 2 traits).
Number of ways to choose the traits ( from Cognition, from each of the other domains):
Out of these chosen traits, exactly are not from the Cognition domain.
The top positions of the result cannot be from Cognition, so they must be filled by of these non-Cognition traits. The number of ways to arrange the top positions is:
The remaining traits (which include the Cognition traits and the unused non-Cognition trait) can be arranged in the bottom positions in ways.
Number of valid results for Case 1 = .
3. Calculate the number of valid results for Case 2 (a non-Cognition domain provides 2 traits).
There are other domains (Influence, Rapport, Planning). Choose to provide traits ( ways).
Number of ways to choose the traits:
Out of these chosen traits, exactly are not from the Cognition domain.
The top positions must be filled by of these non-Cognition traits. The number of ways to arrange the top positions is:
The remaining traits can be arranged in the bottom positions in ways.
Number of valid results for Case 2 = .
4. Sum the results from both cases to find the total number of valid results.
The total number of valid results is the sum of the mutually exclusive results from Case 1 and Case 2:
3,612,672
9(d)
1. Find the total number of unconstrained arrangements for the 5 particular traits.
The 5 particular traits are distinct. The total number of unconstrained arrangements is:
2. Find the number of arrangements where traits from the same domain are consecutive.
If the 2 Rapport traits are consecutive, treat them as a single unit. We have 4 units in total, which can be arranged in ways, and the 2 Rapport traits can be arranged within their unit in ways.
Number of ways =
Similarly, if the 2 Planning traits are consecutive, treat them as a single unit.
Number of ways =
3. Calculate the number of arrangements where both the Rapport traits and the Planning traits are consecutive.
If both the 2 Rapport traits and 2 Planning traits are consecutive, treat each pair as a single unit. There are 3 units in total.
Number of ways =
4. Use the principle of inclusion-exclusion to find the number of valid results.
Using the inclusion-exclusion principle, the number of arrangements with at least one consecutive pair from the same domain is:
The number of valid results with no consecutive traits from the same domain is:
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