In: Operations Management
In the service department of the NAPA (National Auto Parts Association), mechanics requiring parts for auto repair or service present their request forms at the parts department counter. The parts clerk fills a request while the mechanic waits. Mechanics arrive in a random (Poisson) fashion at the rate of 60 per hour, and a clerk can fill requests at the rate of 20 per hour (exponential). If the cost for a parts clerk is $20 per hour and the cost for a mechanic is $50 per hour, answer the following three questions (Because of the high arrival rate, an infinite source may be assumed.)
Arrival rate, = 60 per hour
Service rate, = 20 per hour
Cost for parts clerk, Cs = $ 20 per hour
Cost for mechanic, Cw = $ 50 per hour
/ = 60/20 = 3
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1)
Number of clerks, M = 4
Look for /= 3 and M=4 n the following table:
For /= 3 and M=4, Lq = 1.528
Average number of mechanics in system, L = Lq+/ = 1.528+3 = 4.528
Total cost per hour = M*Cs + L*Cw = 4*20+4.528*50 = $ 306.4
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2)
Using 5 clerks
Look for /= 3 and M=5 n table 18.4
Lq = 0.354
Average number of mechanics in system, L = Lq+/ = 0.354+3 = 3.354
Total cost per hour = M*Cs + L*Cw = 5*20+3.354*50 = $ 267.7
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3)
Using 6 clerks
Look for /= 3 and M=6 n table 18.4
Lq = 0.099
Average number of mechanics in system, L = Lq+/ = 0.099+3 = 3.099
Total cost per hour = M*Cs + L*Cw = 6*20+3.099*50 = $ 275
Total cost per hour for 6 clerks is higher than 5 clerks.
Therefore, there is NO need to have 6 clerks.