Optimization and assignment problems

Questions involving discrete optimization scenarios such as worker-task assignment, route planning, or resource allocation using operations research techniques rather than curve analysis.

8 questions

Edexcel D2 2004 June Q6
6. Joan sells ice cream. She needs to decide which three shows to visit over a three-week period in the summer. She starts the three-week period at home and finishes at home. She will spend one week at each of the three shows she chooses travelling directly from one show to the next. Table 1 gives the week in which each show is held. Table 2 gives the expected profits from visiting each show. Table 3 gives the cost of travel between shows. \begin{table}[h]
\captionsetup{labelformat=empty} \caption{Table 1}
Week123
ShowsA, B, CD, EF, G, H
\end{table} Table 2 \begin{table}[h]
\captionsetup{labelformat=empty} \caption{Table 1}
ShowA\(B\)\(C\)\(D\)\(E\)\(F\)\(G\)\(H\)
Expected Profit \(( \pounds )\)900800100015001300500700600
\end{table} \begin{table}[h]
\captionsetup{labelformat=empty} \caption{Table 3}
Travel costs (£)ABCDEFGH
Home7080150809070
A180150
B140120
C200210
D200160120
E170100110
\end{table} It is decided to use dynamic programming to find a schedule that maximises the total expected profit, taking into account the travel costs.
  1. Define suitable stage, state and action variables.
  2. Determine the schedule that maximises the total profit. Show your working in a table.
  3. Advise Joan on the shows that she should visit and state her total expected profit.
    (3) (Total 18 marks)
Edexcel D2 2006 June Q1
  1. (a) State Bellman's principle of optimality.
    (b) Explain what is meant by a minimax route.
    (c) Describe a practical problem that would require a minimax route as its solution.
    (Total 4 marks)
  2. Three workers, \(P , Q\) and \(R\), are to be assigned to three tasks, 1,2 and 3 . Each worker is to be assigned to one task and each task must be assigned to one worker. The cost, in hundreds of pounds, of using each worker for each task is given in the table below. The cost is to be minimised.
Cost (in \(\pounds 100\) s)Task 1Task 2Task 3
Worker \(P\)873
Worker \(Q\)956
Worker \(R\)1044
Formulate the above situation as a linear programming problem, defining the decision variables and making the objective and constraints clear.
(Total 7 marks)
Edexcel D2 2006 June Q3
3. A college wants to offer five full-day activities with a different activity each day from Monday to Friday. The sports hall will only be used for these activities. Each evening the caretaker will prepare the hall by putting away the equipment from the previous activity and setting up the hall for the activity next day. On Friday evening he will put away the equipment used that day and set up the hall for the following Monday. The 5 activities offered are Badminton (B), Cricket nets (C), Dancing (D), Football coaching (F) and Tennis (T). Each will be on the same day from week to week. The college decides to offer the activities in the order that minimises the total time the caretaker has to spend preparing the hall each week. The hall is initially set up for Badminton on Monday.
The table below shows the time, in minutes, it will take the caretaker to put away the equipment from one activity and set up the hall for the next.
\multirow{3}{*}{}To
Time\(B\)CD\(F\)\(T\)
\(B\)-10815064100
\multirow[t]{4}{*}{From}C108-5410460
D15054-150102
\(F\)64104150-68
\(T\)1006010268-
  1. Explain why this problem is equivalent to the travelling salesman problem. A possible ordering of activities is
    MondayTuesdayWednesdayThursdayFriday
    \(B\)\(C\)\(D\)\(F\)\(T\)
  2. Find the total time taken by the caretaker each week using this ordering.
  3. Starting with Badminton on Monday, use a suitable algorithm to find an ordering that reduces the total time spent each week to less than 7 hours.
  4. By deleting \(B\), use a suitable algorithm to find a lower bound for the time taken each week. Make your method clear.
OCR D2 2006 January Q4
4 Four workers, \(A , B , C\) and \(D\), are to be allocated, one to each of the four jobs, \(W , X , Y\) and \(Z\). The table shows how much each worker would charge for each job.
\includegraphics[max width=\textwidth, alt={}, center]{9c9b1a42-8d16-446a-85a1-4c08e5e368be-3_401_846_1745_642}
  1. What is the total cost of the four jobs if \(A\) does \(W , B\) does \(X , C\) does \(Y\) and \(D\) does \(Z\) ?
  2. Apply the Hungarian algorithm to the table, reducing rows first. Show all your working and explain each step. Give the resulting allocation and the total cost of the four jobs with this allocation.
  3. What problem does the Hungarian algorithm solve?
OCR D2 2011 January Q3
3 The table lists the duration, immediate predecessors and number of workers required for each activity in a project.
ActivityDuration (hours)Immediate predecessorsNumber of workers
\(A\)3-1
\(B\)2-1
C2\(A\)2
\(D\)3\(A\), \(B\)2
E3\(C\)3
\(F\)3C, D3
\(G\)2D3
\(H\)5\(E , F\)1
I4\(F , G\)2
  1. Represent the project by an activity network, using activity on arc. You should make your diagram quite large so that there is room for working.
  2. Carry out a forward pass and a backward pass through the activity network, showing the early event times and late event times clearly at the vertices of your network. State the minimum project completion time and list the critical activities.
  3. Draw a resource histogram to show the number of workers required each hour when each activity begins at its earliest possible start time.
  4. Show how it is possible for the project to be completed in the minimum project completion time when only six workers are available.
OCR D2 2012 January Q2
2 The table lists the durations (in minutes), immediate predecessors and number of workers required for each activity in a project to decorate a room.
ActivityDuration (minutes)Immediate predecessorsNumber of workers
A Cover furniture with dust sheets20-1
B Repair any cracks in the plaster100A1
C Hang wallpaper60B1
D Paint feature wall90B1
\(E\) Paint woodwork120C, D1
\(F\) Put up shelves30C2
G Paint ceiling60A1
\(H\) Clean paintbrushes10\(E , G\)1
I Tidy room20\(F , H\)2
  1. Draw an activity network, using activity on arc, to represent the project. Your network will require a dummy activity.
  2. Carry out a forward pass and a backward pass through the activity network, showing the early event time and the late event time at each vertex of your network. State the minimum project completion time and list the critical activities.
  3. Draw a resource histogram to show the number of workers required at each time when each activity begins at its earliest possible start time. Suppose that there is only one worker available at the start of the project, but another two workers are available later.
  4. Find the latest possible time for the other workers to start and still have the project completed on time. Which activities could happen at the same time as painting the ceiling if the other two workers arrive at this latest possible time?
    [0pt] [Do not change your resource histogram from part (iii).]
OCR D2 2009 June Q2
2
  1. Set up a dynamic programming tabulation to find the maximum weight route from ( \(0 ; 0\) ) to ( \(3 ; 0\) ) on the following directed network.
    \includegraphics[max width=\textwidth, alt={}, center]{9057da95-c53a-416c-8340-c94aff366385-3_595_1056_404_587} Give the route and its total weight.
  2. The actions now represent the activities in a project and the weights represent their durations. This information is shown in the table below.
    ActivityDurationImmediate predecessors
    \(A\)8-
    \(B\)9-
    C7-
    D5\(A\)
    E6\(A\)
    \(F\)4\(B\)
    \(G\)5B
    \(H\)6\(B\)
    \(I\)10C
    \(J\)9\(C\)
    \(K\)6\(C\)
    \(L\)7D, F, I
    \(M\)6\(E , G , J\)
    \(N\)8\(H\), \(K\)
    Make a large copy of the network with the activities \(A\) to \(N\) labelled appropriately. Carry out a forward pass to find the early event times and a backward pass to find the late event times. Find the minimum completion time for the project and list the critical activities.
  3. Compare the solutions to parts (i) and (ii).
Edexcel D2 Q4
4. A furniture manufacturer has three workshops, \(W _ { 1 } , W _ { 2 }\) and \(W _ { 3 }\). Orders for rolls of fabric are to be placed with three suppliers, \(S _ { 1 } , S _ { 2 }\) and \(S _ { 3 }\). The supply, demand and cost per roll in pounds, according to which supplier each workshop uses, are given in the table below.
\(W _ { 1 }\)\(W _ { 2 }\)\(W _ { 3 }\)Available
\(S _ { 1 }\)12111730
\(S _ { 2 }\)751025
\(S _ { 3 }\)56810
Required201530
Starting with the north-west corner method of finding an initial solution, find an optimal transportation pattern which minimises the total cost. State the final solution and its total cost.
(11 marks)