D2 June 2011 Q6
6. Three workers, P, Q and R, are to be assigned to three tasks, A, B and C. Each worker must be assigned to just one task and each task must be assigned to just one worker.
Table 1 shows the cost of using each worker for each task. The total cost is to be minimised.
| Task A | Task B | Task C | |
|---|---|---|---|
| Worker P | 27 | 31 | 25 |
| Worker Q | 26 | 30 | 34 |
| Worker R | 35 | 29 | 32 |
Table 1
You are not required to solve the problem. (7)
Table 2 shows the profit gained by using each worker for each task. The total profit is to be maximised.
| Task A | Task B | Task C | |
|---|---|---|---|
| Worker P | 33 | 37 | 31 |
| Worker Q | 32 | 36 | 40 |
| Worker R | 41 | 35 | 38 |
Table 2
| Scheme | Marks | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||||||||
| Let \(x_{ij} = \begin{cases} 1 & \text{if worker does the task}\\ 0 & \text{otherwise}\end{cases}\) | B1 | ||||||||||||||||
| Where \(x_{ij}\) indicates worker i being assigned to task j, \(i \in \{P, Q, R\}\), \(j \in \{A, B, C\}\) | B1 | ||||||||||||||||
| Minimise \(27x_{PA} + 31x_{PB} + 25x_{PC} + 26x_{QA} + 30x_{QB} + 34x_{QC} + 35x_{RA} + 29x_{RB} + 32x_{RC}\) | B1 B1 | ||||||||||||||||
| Subject to: \(x_{PA} + x_{PB} + x_{PC} = 1\) \(x_{QA} + x_{QB} + x_{QC} = 1\) \(x_{RA} + x_{RB} + x_{RC} = 1\) \(x_{PA} + x_{QA} + x_{RA} = 1\) \(x_{PB} + x_{QB} + x_{RB} = 1\) \(x_{PC} + x_{QC} + x_{RC} = 1\) | M1 A1 A1 | ||||||||||||||||
| (7) |
Notes
1B1: defining variables
2B1: defining variables
3B1: minimise
4B1: cao
1M1: At least 3 equations, coefficients of 1. Accept inequalities here should be precisely 9 variables.
1A1: cao 3 equations correct accept slack variables if defined
2A1: cao 6 equations correct accept slack variables if defined
| Scheme | Marks | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Since we need to maximise first subtract all entries from some \(n \geqslant 41\) | M1 | ||||||||||||||||
| A1 | ||||||||||||||||
| (2) | |||||||||||||||||
| (9 marks) |
Notes
1M1: subtracting from some \(n \geqslant 41\) condone up to two errors
1A1: correct