1. IE 417: Operations Research HOT DOG KING RESTAURANT Extra Credit Chapter 20 section 5, #2 By: Abby Almonte Crystal Chea Anthony Yoohanna

2. Overview • Problem Statement • Assumptions • M/M/1/GD/C/∞ • Manual Solutions • WinQSB Solutions • Extra Questions • Sensitivity Analysis • Report to Manager

3. Problem Statement An average of 40 cars per hour which are exponentially distributed want to use the drive-in at the Hot Dog King Restaurant. If more than 4 cars are in the line, including the car at the window, a car will not enter the line. It takes an average of 4 minutes to serve a car. a) What is the average number of cars waiting for the drive- in window? (not including the car at the window) b) On the average, how many cars will be served per hour? c) If a customer just joined the line to the drive-in window, on average how long will it be until he or she has received their food?

4. Assumptions • The information in the problem is accurate • The queuing problem is a M/M/1/GD/C/∞problem

5. M/M/S/GD/C/∞ Type of System Arrival Process Service Process General Queue Discipline Customer Population Exponential Distribution Number of Servers Limit Capacity

6. M/M/1/GD/4/∞ Type of System Exponential Distribution General Queue Discipline Customer Population Number of Servers Limit Capacity FOR EXAMPLE…

7. Manual Solutions • Make pre-calculations • Identify Arrival Rate (λ) • Calculate Effective Arrival Rate (λe) • Identify Service Rate (μ) • Calculate rho (ρ) = λ/μ

8. Pre-Calculations • Given that there are an average of 40 cars per hour to use the drive-in λ = 40 cars per hour • BUT there can only be 4 cars in the drive-in, therefore the effective • arrival rate is: λe = λ ( 1 – π4) = 40 (1 – 0.62967) = 14.9 cars per hour • Then, we calculate service rate by using the given average service • time which is 4 minutes per car. μ = 60 minute = 15 cars per hour 4 minutes per car • Using λ & μ values, we find ρ λ = 40 cars per hour = 2.667 μ 15 cars per hour

9. Part A a) What is the average number of cars waiting for the drive- in window? (not including the car at the window) Lq= L – Ls We use this equation which represents the number of customers in the queue with respect to an M/M/1/GD/∞ system

10. Part A continued. Equation • Begin by finding L Lq= L – Ls L = ρ [ 1 – (C + 1) ρC + CρC+1] (1 – ρC+1) ( 1 – ρ) L = 2.667 [ 1 – (4 + 1) 2.6674 + 4(2.667 4 + 1)] = ( 1 – 2.6674 + 1) ( 1 – 2.667) 3.437 • Then Ls Ls = 1 – π0 Π0 = _( 1 – ρ )_ ( 1 – ρ C + 1)   = _( 1 – 2.667)_ = ( 1 – 2.667 4 + 1 ) 0.012 Ls = 1 – 0.012 = 0.988

11. Part A continued. • Finally Lq Lq= L – Ls = 3.437 – 0.0988 = 2.449 Solution: About 2.5 cars

12. Part B b) On the average, how many cars will be served per hour? π4= (ρ4)(π0) • First we find the probability of having 4 cars in the drive-in, which • is noted as π4 Π4 = ρ4 π0 Π4 = 2.6674 (0.012452) = 0.62967 Refer to Part A

13. Part B continued. • Then λ ( 1 – π4) = 40 (1 – 0.62967) = 14.8132 Solution: About 14.8 cars per hour

14. Part C c) If a customer just joined the line to the drive-in window, on average how long will it be until he or she has received their food? C = 4 π4 Refer to Part B W = ___L___ λ(1 – πc) We use this equation which represents time a customer spends in the system. W = ___3.43709___ = 40 ( 1- 0.629675) 0.23203 hours Solution: About 13.9 minutes

15. Using WinQSB • The red box indicates the cost values we decided to apply in WinQSB to • calculate hourly cost.

16. Output: WinQSB

17. Probability of Customers • This table represents the probability of having the number of customers in the line. Since there is a capacity of 4 customers and an arrival rate of 40 customers per hour, the table shows that having 4 customers in line has a higher probability compared to having zero.

18. Manual Solutions vs. WinQSB

19. Questions • What happens if the average service time changes? • What happens if the arrival rate changes? • What happens if the number of servers changes? • What can be done to reduce the hourly cost? • What is the best option for reducing the hourly cost? Initiate SENSITIVITY ANALYSIS

20. Sensitivity Analysis: Number of Servers Servers 1 - 5 • Average balked customers drops to nearly zero • Total hourly cost of the system drops to $116.44 • From 1 server to 2 the cost drops the most.

21. Sensitivity Analysis: Number of Servers Servers 1 - 7 • More than 5 servers results in a gain of cost.

22. Sensitivity Analysis: Service Rate • From increasing your service rate from 15 cars to 25 cars per hour, the total cost is still higher than increasing the number of servers.

23. Sensitivity Analysis: Arrival Rate • From increasing your service rate from 15 cars to 25 cars per hour, the total cost is still higher due to the amount of customers balking.

24. Report to Manager Original Data • The total hourly cost is $529.69 • The total hourly cost from balking is $453.36 Sensitivity Analysis • Add up to 4 servers to reduce total hourly cost significantly • Increased service rate has small effect on total hourly cost • If arrival rate decreases, so will total hourly cost (not ideal)

25. Questions?