1. Customers arrive to the checkout area of New Navy a rate of 80 customers per hour. The inter- arrival times are exponentially distributed. At the checkout area, the service time of each customer is exponentially distributed, and on average each cashier can serve 10 customers per hour. The store has hired 12 cashiers to checkout customers and the checkout area consists of a single pool of cashiers with one common line in front of them. a. What is the probability that a customer's service time is less than 12 minutes? b. How long do customers have to wait in the queue on average, in seconds? Please use the La table (not the M/M/s spreadsheet), and show your work. c. What is the average total number of customers in the checkout area? This includes the customers who are waiting to be checked out and the customers being checked out. Please use the Lg table (not the M/M/s spreadsheet), and show your work. d. New Navy is considering having 13 cashiers instead of 12. An extra cashier will cost an extra $10/hour. New Navy believes that for each second that the average waiting time in the queue improves, their revenue will increase by $1/hour. (Note that this improvement is in total, not per customer.) Should they hire an extra cashier to have 13 instead of 12? (You can use the MMS spreadsheet for this question, but please explain your answer.) e. Suppose now that New Navy decides to replace the single common line for its customers with 12 separate lines, one line in front of each of the 12 cashiers. What is now the average number of customers waiting to be checked out? This only includes the customers who are waiting to be checked out (not the customers being served).

A First Course in Probability (10th Edition)
10th Edition
ISBN:9780134753119
Author:Sheldon Ross
Publisher:Sheldon Ross
Chapter1: Combinatorial Analysis
Section: Chapter Questions
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1. Customers arrive to the checkout area of New Navy a rate of 80 customers per hour. The inter-
arrival times are exponentially distributed. At the checkout area, the service time of each
customer is exponentially distributed, and on average each cashier can serve 10 customers per
hour. The store has hired 12 cashiers to checkout customers and the checkout area consists of
a single pool of cashiers with one common line in front of them.
a. What is the probability that a customer's service time is less than 12 minutes?
b. How long do customers have to wait in the queue on average, in seconds? Please use the La
table (not the M/M/s spreadsheet), and show your work.
c. What is the average total number of customers in the checkout area? This includes the
customers who are waiting to be checked out and the customers being checked out. Please use
the La table (not the M/M/s spreadsheet), and show your work.
d. New Navy is considering having 13 cashiers instead of 12. An extra cashier will cost an extra
$10/hour. New Navy believes that for each second that the average waiting time in the queue
improves, their revenue will increase by $1/hour. (Note that this improvement is in total, not per
customer.) Should they hire an extra cashier to have 13 instead of 12? (You can use the MMS
spreadsheet for this question, but please explain your answer.)
e. Suppose now that New Navy decides to replace the single common line for its customers with
12 separate lines, one line in front of each of the 12 cashiers. What is now the average number of
customers waiting to be checked out? This only includes the customers who are waiting to be
checked out (not the customers being served).
Transcribed Image Text:1. Customers arrive to the checkout area of New Navy a rate of 80 customers per hour. The inter- arrival times are exponentially distributed. At the checkout area, the service time of each customer is exponentially distributed, and on average each cashier can serve 10 customers per hour. The store has hired 12 cashiers to checkout customers and the checkout area consists of a single pool of cashiers with one common line in front of them. a. What is the probability that a customer's service time is less than 12 minutes? b. How long do customers have to wait in the queue on average, in seconds? Please use the La table (not the M/M/s spreadsheet), and show your work. c. What is the average total number of customers in the checkout area? This includes the customers who are waiting to be checked out and the customers being checked out. Please use the La table (not the M/M/s spreadsheet), and show your work. d. New Navy is considering having 13 cashiers instead of 12. An extra cashier will cost an extra $10/hour. New Navy believes that for each second that the average waiting time in the queue improves, their revenue will increase by $1/hour. (Note that this improvement is in total, not per customer.) Should they hire an extra cashier to have 13 instead of 12? (You can use the MMS spreadsheet for this question, but please explain your answer.) e. Suppose now that New Navy decides to replace the single common line for its customers with 12 separate lines, one line in front of each of the 12 cashiers. What is now the average number of customers waiting to be checked out? This only includes the customers who are waiting to be checked out (not the customers being served).
Expected Number of People Waiting in Line (L) for Various Values of S and X/μ
1
0.10
0.0111
0.15 0.0264 0.0006
0.20 0.0500 0.0020
1.0
1.2
14
1.6
1.8
20
0.25 0.0833 0.0039
0.30 0.1285 0.0069
0.35 0.1884 0.0110
0.40 0.2666 0.0166
0.45 0.3681 0.0239 0.0019
0.50 0.5000 0.0333 0.0030
0.55 0.6722 0.045 0.0043
0.60 0.9090 0.0593 0.0061
0.65 12071 0.0767 0.0084
0.70 1.6333 0.0976 0.0112
0.75 2.2500 0.1227 0.0147
0.80 3.2000 0.1523 0.0189
0.85 4.8165 0.1873 0.0239 0.0031
0.90 8.1000 0.2285 0.0300 0.0041
0.95 18.0500 0.2767 0.0371 0.0053
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
40
42
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.6
6.8
7.0
7.2
74
7.6
7.8
8.0
8.2
84
86
8.8
9.0
92
94
9.6
2
9.8
10
3
4
5
0.3333 0.0454 0.0067
0.6748 0.0940 0.0158
13449 0.1778 0.0324 0.0059
2.8441 0.3128 0.0604 0.0121
7.6731
NUMBER OF SERVICE CHANNELS, S
7
6
0.5320 0.1051 0.0227 0.0047
0.0090
0.8888 0.1730 0.0390
0.0158
1.4907 0.2770 0.066
2.1261
4.9322
12.2724
0.4205 0.1047 0.0266 0.0065
0.6581 0.1609 0.0425 0.0110
1.0000 0.2411 0.0659 0.0180
8
1.5282 0.3541 0.0991 0.0282 0.0077
2.3855 0.5128 0.1452 0.0427 0.0122
3.9060 0.7365 0.2085 0.0631
0.0189
7.0893 1.0550
0.2947 0.0912
0.0283
16.9366 1.5181 0.4114 0.1292
0.0412
0.5694 0.1801
0.0590
2.2164
3.3269
5.2675
0.7837 0.2475 0.0827
1.0777 0.3364 0.1142
9.2885 1.4857 0.4532 0.1555
21.6384 2.0708 0.6071 0.2092
9
0.0084
0.0127
0.0189
0.0273
0.0389
0.0541
0.0742
4.4471 1.3471
6.3133 1.7288
9.5102
2.2324
16.0379
2.9113
35.8956
3.8558
10
0.0087
0.0128
0.0184
0.0260
5.2264
7.3441
11
1.6364
2.0736
10.9592
2.6470
18.3223 3.4160
40.6824 4.4805
0.1006
0.0361 0.0125
0.1345
0.0492 0.0175
2.9375 0.8102 0.2785
43004 1.0804 0.3680
6.6609 1.4441 0.5871 0.1779 0.0663 0.0243 0.0085
11.5178 1.9436 0.6313 0.2330 0.0683 0.0330 0.0119
26.3726 2.6481 0.8225
0.3032 0.1164 0.0443 0.0164
3.6878 1.0707 0.3918 0.1518 0.0590 0.0224
5.2979 1.3967 0.5037 0.1964
8.0768 1.8040 0.6454 0.2524 0.1008 0.0398 0.0153
13.7992 2.4198 0.8247 0.3222 0.1302 0.0523 0.0205
31.1270 3.2441 1.0533
0.0775 0.0300 0.0113
12
exhibit 7A.9
13
0.4090 0.1666 0.0679 0.0271 0.0105
0.5172 0.2119 0.0876
0.6521 0.2677 0.1119
0.8202 0.3364 01420
1.0310 0.4211 0.1789
1.2972 0.5250 0.2243
45.4769 5.1156
6.8210
14
0.0357 0.0141
0.0463 0.0187
15
0.0595 0.0245 0.0097
0.0761 0.0318 0.0129
0.0966 0.0410 0.0168
0.6530 0.2796 0.1214 0.0522 0.0220
0.8109 03469 0.1520 0.0663 0.0283
10060 0.4288 0.1891 0.0834 0.0361
1.2484 0.5236 0.2341 0.1043 0.0459
1.5524 0.6501 0.2885 0.1208 0.0577
6.0183
1.9366 0.7980 0.3543 0.1603 0.0723
8.3869
24293 0.9788 0.4333 0.1974 0.0899
12.4183
3.0732 1.2010 0.5267 0.2419 01111
20.6160 3.9318 1.4752 0.5437 0.2952 01367
1.8165 0.7827 0.3699 0.16731
2.2465 0.9506 0.4352 0.2040
253
Transcribed Image Text:Expected Number of People Waiting in Line (L) for Various Values of S and X/μ 1 0.10 0.0111 0.15 0.0264 0.0006 0.20 0.0500 0.0020 1.0 1.2 14 1.6 1.8 20 0.25 0.0833 0.0039 0.30 0.1285 0.0069 0.35 0.1884 0.0110 0.40 0.2666 0.0166 0.45 0.3681 0.0239 0.0019 0.50 0.5000 0.0333 0.0030 0.55 0.6722 0.045 0.0043 0.60 0.9090 0.0593 0.0061 0.65 12071 0.0767 0.0084 0.70 1.6333 0.0976 0.0112 0.75 2.2500 0.1227 0.0147 0.80 3.2000 0.1523 0.0189 0.85 4.8165 0.1873 0.0239 0.0031 0.90 8.1000 0.2285 0.0300 0.0041 0.95 18.0500 0.2767 0.0371 0.0053 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 40 42 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.6 6.8 7.0 7.2 74 7.6 7.8 8.0 8.2 84 86 8.8 9.0 92 94 9.6 2 9.8 10 3 4 5 0.3333 0.0454 0.0067 0.6748 0.0940 0.0158 13449 0.1778 0.0324 0.0059 2.8441 0.3128 0.0604 0.0121 7.6731 NUMBER OF SERVICE CHANNELS, S 7 6 0.5320 0.1051 0.0227 0.0047 0.0090 0.8888 0.1730 0.0390 0.0158 1.4907 0.2770 0.066 2.1261 4.9322 12.2724 0.4205 0.1047 0.0266 0.0065 0.6581 0.1609 0.0425 0.0110 1.0000 0.2411 0.0659 0.0180 8 1.5282 0.3541 0.0991 0.0282 0.0077 2.3855 0.5128 0.1452 0.0427 0.0122 3.9060 0.7365 0.2085 0.0631 0.0189 7.0893 1.0550 0.2947 0.0912 0.0283 16.9366 1.5181 0.4114 0.1292 0.0412 0.5694 0.1801 0.0590 2.2164 3.3269 5.2675 0.7837 0.2475 0.0827 1.0777 0.3364 0.1142 9.2885 1.4857 0.4532 0.1555 21.6384 2.0708 0.6071 0.2092 9 0.0084 0.0127 0.0189 0.0273 0.0389 0.0541 0.0742 4.4471 1.3471 6.3133 1.7288 9.5102 2.2324 16.0379 2.9113 35.8956 3.8558 10 0.0087 0.0128 0.0184 0.0260 5.2264 7.3441 11 1.6364 2.0736 10.9592 2.6470 18.3223 3.4160 40.6824 4.4805 0.1006 0.0361 0.0125 0.1345 0.0492 0.0175 2.9375 0.8102 0.2785 43004 1.0804 0.3680 6.6609 1.4441 0.5871 0.1779 0.0663 0.0243 0.0085 11.5178 1.9436 0.6313 0.2330 0.0683 0.0330 0.0119 26.3726 2.6481 0.8225 0.3032 0.1164 0.0443 0.0164 3.6878 1.0707 0.3918 0.1518 0.0590 0.0224 5.2979 1.3967 0.5037 0.1964 8.0768 1.8040 0.6454 0.2524 0.1008 0.0398 0.0153 13.7992 2.4198 0.8247 0.3222 0.1302 0.0523 0.0205 31.1270 3.2441 1.0533 0.0775 0.0300 0.0113 12 exhibit 7A.9 13 0.4090 0.1666 0.0679 0.0271 0.0105 0.5172 0.2119 0.0876 0.6521 0.2677 0.1119 0.8202 0.3364 01420 1.0310 0.4211 0.1789 1.2972 0.5250 0.2243 45.4769 5.1156 6.8210 14 0.0357 0.0141 0.0463 0.0187 15 0.0595 0.0245 0.0097 0.0761 0.0318 0.0129 0.0966 0.0410 0.0168 0.6530 0.2796 0.1214 0.0522 0.0220 0.8109 03469 0.1520 0.0663 0.0283 10060 0.4288 0.1891 0.0834 0.0361 1.2484 0.5236 0.2341 0.1043 0.0459 1.5524 0.6501 0.2885 0.1208 0.0577 6.0183 1.9366 0.7980 0.3543 0.1603 0.0723 8.3869 24293 0.9788 0.4333 0.1974 0.0899 12.4183 3.0732 1.2010 0.5267 0.2419 01111 20.6160 3.9318 1.4752 0.5437 0.2952 01367 1.8165 0.7827 0.3699 0.16731 2.2465 0.9506 0.4352 0.2040 253
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