csatmedium

There are 5 tasks and 5 persons. Task-1 cannot be assigned to either person-1 or person-2. Task-2 must be assigned to either person-3 or person-4. Every person is to be assigned one task. In how many ways can the assignment be done?

  1. A.6
  2. B.12
  3. C.24
  4. D.144
▶ Answer & Explanation

Correct answer: C. 24

Let's analyze the constraints. For Task 2, there are 2 choices (Person 3 or Person 4). Once Task 2 is assigned, we consider Task 1. If Person 3 was assigned Task 2, then Person 4 is still available for Task 1. If Person 4 was assigned Task 2, then Person 3 is still available for Task 1. In either case, Task 1 can be assigned to Person 3 or Person 4 (if not already assigned to Task 2) or Person 5. However, a more direct approach considers the total permutations and then subtracts invalid ones, or builds valid assignments step-by-step. Considering the constraints, let's assign Task 2 first (2 options). Then Task 1 has 3 available persons (P3, P4, P5, since P1, P2 are excluded, and the person assigned to T2 is also unavailable). The remaining 3 tasks can be assigned to the remaining 3 persons in 3! ways. So, 2 (for T2) * 3 (for T1) * 3! (for remaining) = 2 * 3 * 6 = 36. Wait, this is not matching the options. Let's re-evaluate. Another approach: Total permutations of 5 tasks to 5 people is 5! = 120. Let's break it down by cases. Case 1: T2 is assigned to P3. Then T1 can be assigned to P4 or P5 (2 options). The remaining 3 tasks can be assigned to the remaining 3 people in 3! ways. So, 1 * 2 * 3! = 12. Case 2: T2 is assigned to P4. Then T1 can be assigned to P3 or P5 (2 options). The remaining 3 tasks can be assigned to the remaining 3 people in 3! ways. So, 1 * 2 * 3! = 12. Total ways = 12 + 12 = 24.

Source: UPSC csat 2015

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