Linear Programming Problems

Linear Programming is a mathematical technique for optimum allocation of limited or scarce resources, such as labour, material, machine, money, energy and so on , to several competing activities such as products, services, jobs and so on, on the basis of a given criteria of optimality.

The term ‘Linear’ is used to describe the proportionate relationship of two or more variables in a model. The given change in one variable will always cause a resulting proportional change in another variable.

The word , ‘Programming’ is used to specify a sort of planning that involves the economic allocation of limited resources by adopting a particular course of action or strategy among various alternatives strategies to achieve the desired objective.

Hence, Linear Programming is a mathematical technique for optimum allocation of limited or scarce resources, such as labour, material, machine, money energy etc.

Structure of Linear Programming model

The general structure of the Linear Programming model essentially consists of three components.

i) The activities (variables) and their relationships

ii) The objective function and

iii) The constraints

The activities are represented by X1 X2, X3 ……..Xn.

These are known as Decision variables.

The objective function of an LPP (Linear Programming Problem) is a mathematical representation of the objective in terms a measurable quantity such as profit, cost, revenue, etc.

Optimize (Maximize or Minimize) Z=C1X1 +C2X2+ ………..Cn Xn Where Z is the measure of performance variable

X1, X2, X3, X4…..Xn are the decision variables

C1, C2, …Cn are the parameters that give contribution to decision variables.

The constraints: These are the set of linear inequalities and/or equalities which impose restriction of the limited resources

Assumptions of Linear Programming Certainty

In all LP models it is assumed that, all the model parameters such as availability of resources, profit (or cost) contribution of a unit of decision variable and consumption of resources by a unit of decision variable must be known and constant.

Divisibility (Continuity)

The solution values of decision variables and resources are assumed to have either whole numbers (integers) or mixed numbers (integer or fractional). However, if only integer variables are desired, then Integer programming method may be employed.

Additivity

The value of the objective function for the given value of decision variables and the total sum of resources used, must be equal to the sum of the contributions (Profit or Cost) earned from each decision variable and sum of the resources used by each decision variable respectively. /The objective function is the direct sum of the individual contributions of the different variables

Linearity

All relationships in the LP model (i.e. in both objective function and constraints) must be linear. (Means that there are constant returns to scale – and there are no economies of scale)

Finite choices: An LP model assumes that number of choices available to the decision maker is limited and decision variables do not assume negative values.

General Mathematical Model of an LPP

Guidelines for formulating Linear Programming model

i) Identify and define the decision variable of the problem

ii) Define the objective function

iii) State the constraints to which the objective function should be optimized

(i.e. Maximization or Minimization)

iv) Add the non-negative constraints from the consideration that the negative values of the decision variables do not have any valid physical interpretation

Example 1.

A firm is engaged in producing two products. A and B. Each unit of product A requires 2 kg of raw material and 4 labour hours for processing, where as each unit of B requires 3 kg of raw materials and 3 labour hours for the same type. Every week, the firm has an availability of 60 kg of raw material and 96 labour hours. One unit of product A sold yields Rs.40 and one unit of product B sold gives Rs.35 as profit.

Formulate this as an Linear Programming Problem to determine as to how many units of each of the products should be produced per week so that the firm can earn maximum profit.

i) Identify and define the decision variable of the problem

Let X1 and X2 be the number of units of product A and product B produced per week.

ii) Define the objective function

Since the profits of both the products are given, the objective function is to maximize the profit.

Max Z = 40 X1 + 35 X2

iii) State the constraints to which the objective function should be optimized (i.e. Maximization or Minimization)

There are two constraints one is raw material constraint and the other one is labour constraint..

Example 2.

A manufacturer produces two types of models M1 and M2.Each model of the type M1 requires 4 hours of grinding and 2 hours of polishing; where as each model of M2 requires 2 hours of grinding and 5 hours of polishing. The manufacturer has 2 grinders and 3 polishers. Each grinder works for 40 hours a week and each polisher works 60 hours a week. Profit on M1 model is Rs.3.00 and on model M2 is Rs.4.00.Whatever produced in a week is sold in the market. How should the manufacturer allocate his production capacity to the two types of models, so that he makes maximum profit in a week?

i) Identify and define the decision variable of the problem Let X1 and X2 be the number of units of M1 and M2 model.

ii) Define the objective function

Since the profits on both the models are given, the objective function is to maximize the profit.

Max Z = 3 X1 + 4 X2

iii) State the constraints to which the objective function should be optimized (i.e.

Maximization or Minimization)

There are two constraints one for grinding and the other for polishing. The grinding constraint is given by

4 X1 + 2 X2 < 80

No of hours available on grinding machine per week is 40 hrs. There are two grinders.

Hence the total grinding hour available is 40 X 2 = 80 hours. The polishing constraint is given by

2 X1 + 5 X2 < 180

No of hours available on polishing machine per week is 60 hrs. There are three grinders.

Hence the total grinding hour available is 60 X 3 = 180 hours. Finally we have,

Example 3

The agricultural research institute suggested the farmer to spread out at least 4800 kg of special phosphate fertilizer and not less than 7200 kg of a special nitrogen fertilizer to raise the productivity of crops in his fields. There are two sources for obtaining these

– mixtures A and mixtures B. Both of these are available in bags weighing 100kg each and they cost Rs.40 and Rs.24 respectively. Mixture A contains phosphate and nitrogen equivalent of 20kg and 80 kg respectively, while mixture B contains these ingredients equivalent of 50 kg each. Write this as an LPP and determine how many bags of each type the farmer should buy in order to obtain the required fertilizer at minimum cost.

i) Identify and define the decision variable of the problem

Let X1 and X2 be the number of bags of mixture A and mixture B.

ii) Define the objective function

The cost of mixture A and mixture B are given; the objective function is to minimize the cost

iii) State the constraints to which the objective function should be optimized. The above objective function is subjected to following constraints.

20 X1+ 50 X2 >4800 Phosphate requirement

80 X1 + 50 X2 >7200 Nitrogen requirement

X1, X2 >0

Finally we have,

Min.Z = 40 X1 + 24 X2

is subjected to three constraints

20 X1 + 50 X2 >4800

80 X1+ 50 X2 >7200

X1, X2 >0

Example 4.

A firm can produce 3 types of cloth, A , B and C.3 kinds of wool are required Red, Green and Blue.1 unit of length of type A cloth needs 2 meters of red wool and 3 meters of blue wool.1 unit of length of type B cloth needs 3 meters of red wool, 2 meters of green wool and 2 meters of blue wool.1 unit type of C cloth needs 5 meters of green wool and 4 meters of blue wool. The firm has a stock of 8 meters of red, 10 meters of green and 15 meters of blue. It is assumed that the income obtained from 1 unit of type A is Rs.3, from B is Rs.5 and from C is Rs.4.Formulate this as an LPP.( December2005/January 2006)

i) Identify and define the decision variable of the problem

Let X1, X2 and X3 are the quantity produced of cloth type A,B and C respectively.

ii) Define the objective function

The incomes obtained for all the three types of cloths are given; the objective function is to maximize the income.

Max Z = 3 X1 + 5 X2 + 4 X3

iii) State the constraints to which the objective function should be optimized.

The above objective function is subjected to following three constraints.

Example 5.

A Retired person wants to invest upto an amount of Rs.30,000 in fixed income securities. His broker recommends investing in two Bonds: Bond A yielding 7% and Bond B yielding 10%. After some consideration, he decides to invest at most of Rs.12,000 in bond B and atleast Rs.6,000 in Bond A. He also wants the amount invested in Bond A to be atleast equal to the amount invested in Bond B. What should the broker recommend if the investor wants to maximize his return on investment? Solve graphically. (January/ February 2004)

Minimization problems

Example 6.

A person requires 10, 12, and 12 units chemicals A, B and C respectively for his garden. A liquid product contains 5, 2 and 1 units of A,B and C respectively per jar. A dry product contains 1,2 and 4 units of A,B and C per carton.

If the liquid product sells for Rs.3 per jar and the dry product sells for Rs.2 per carton, how many of each should be purchased, in order to minimize the cost and meet the requirements?

i) Identify and define the decision variable of the problem

Let X1 and X2 be the number of units of liquid and dry products.

ii) Define the objective function

The cost of Liquid and Dry products are given ; the objective function is to minimize the cost

iii) State the constraints to which the objective function should be optimized.

The above objective function is subjected to following three constraints. 5 X1 + X2 >10

2 X1 + 2 X2 >12

X1 + 4 X2 >12

X1, X2 >0

Finally we have,

Min. Z = 3 X1 + 2 X2

is subjected to three constraints

5 X1 + X2 >10

2 X1 + 2 X2 >12

X1 + 4 X2 >12

X1, X2 >0

Example 7.

A Scrap metal dealer has received a bulk order from a customer for a supply of atleast 2000 kg of scrap metal. The consumer has specified that atleast 1000 kgs of the order must be high quality copper that can be melted easily and can be used to produce tubes. Further, the customer has specified that the order should not contain more than 200 kgs of scrap which are unfit for commercial purposes. The scrap metal dealer purchases the scrap from two different sources in an unlimited quantity with the following percentages (by weight) of high quality of copper and unfit scrap

The cost of metal purchased from source A and source B are Rs.12.50 and Rs.14.50 per kg respectively. Determine the optimum quantities of metal to be purchased from the two sources by the metal scrap dealer so as to minimize the total cost (February 2002)

i) Identify and define the decision variable of the problem

Let X1 and X2 be the quantities of metal to be purchased from the two sources A and B.

ii) Define the objective function

The cost of metal to be purchased by the metal scrap dealer are given; the objective function is to minimize the cost

Min. Z = 12.5 X1 + 14.5 X2

iii) State the constraints to which the objective function should be optimized.

The above objective function is subjected to following three constraints. X1 + X2 >2,000

0.4 X1 + 0.75 X2 >1,000

0.075 X1 + 0.1 X2 + 4X3 < 200

X1, X2 >0

Example 8.

A farmer has a 100 acre farm. He can sell all tomatoes, lettuce or radishes and can raise the price to obtain Rs.1.00 per kg. for tomatoes , Rs.0.75 a head for lettuce and Rs.2.00 per kg for radishes. The average yield per acre is 2000kg.of tomatoes, 3000 heads of lettuce and 1000 kgs of radishes. Fertilizers are available at Rs.0.50 per kg and the amount required per acre is 100 kgs for each tomatoes and lettuce and 50kgs for radishes. Labour required for sowing, cultivating and harvesting per acre is 5 man-days for tomatoes and radishes and 6 man-days for lettuce. A total of 400 man-days of labour are available at Rs.20.00 per man-day. Formulate this problem as LP model to maximize the farmers profit.

Example 9.

An electronics company produces three types of parts for automatic washing machines .It purchases castings of the parts from a local foundry and then finishes the part on drilling, shaping and polishing machines. The selling prices of parts A, B, and C respectively are Rs 8, Rs.10 and Rs.14.All parts made can be sold. Castings for parts A, B and C respectively cost Rs.5, Rs.6 and Rs.10.

The shop possesses only one of each type of machine. Cost per hour to run each of the three machines are Rs.20 for drilling, Rs.30 for shaping and Rs.30 for polishing. The capacities (parts per hour) for each part on each machine are shown in the following table.

The management of the shop wants to know how many parts of each type it should produce per hour in order to maximize profit for an hour’s run. Formulate this problem as an LP model so as to maximize total profit to the company.

i) Identify and define the decision variable of the problem

Let X1 and X2 and X3 be the number of types A, B and C parts produced per hour respectively .

ii) Define the objective function

With the information given, the hourly profit for part A, B, and C would be as follows Profit per type A part = (8 – 5) – (20/25 +30/25 + 30/40) = 0.25

Profit per type B part = (10 – 6) – (20/40 + 30/20 + 30/30) = 1 Profit per type C part = (14 – 10) – (20/25 + 30/20 + 30/40) = 0.95 Then,

Maximize Z = 0.25 X1 + 1 X2 + 0.95 X3

iii) State the constraints to which the objective function should be optimized.

Example 10.

A city hospital has the following minimal daily requirements for nurses.

Nurses report at the hospital at the beginning of each period and work for 8 consecutive hours. The hospital wants to determine the minimal number of nurses to be employed so that there will be a sufficient number of nurses available for each period.

Formulate this as a linear programming problem by setting up appropriate constraints and objective function.

i) Identify and define the decision variable of the problem

Let X1, X2, X3, X4, X5 and X6 be the number of nurses joining duty at the beginning of periods 1, 2, 3, 4, 5 and 6 respectively.

ii) Define the objective function

Minimize Z = X1 + X2 + X3 + X4 + X5 + X6

iii) State the constraints to which the objective function should be optimized. The above objective function is subjected to following constraints.

Example 11.The marketing department of Everest Company has collected information on the problem of advertising for its products. This relates to the advertising media available, the number of families expected to be reached with each alternative, cost per advertisement, the maximum availability of each medium and the expected exposure of each one (measured as the value of one advertisement in each of the media):

The information is as given as here:

Other information and requirements

a) The advertisement budget is Rs.70,000

b) At least 40,000 families should be covered

c) At least 2 insertions is given in Sunday edition daily but not more than 4 ads should be given on the TV

Draft this as a linear programming problem. The company’s objective is to maximize the expected exposure.

i) Identify and define the decision variable of the problem

Let X1, X2, X3, and X4, are number ads on TV, number ads on radio, number ads in Sunday edition of a daily, and number ads in a magazine respectively.