U.S. Postal Service standards call for overnight delivery within a zone of about

Question

U.S. Postal Service standards call for overnight delivery within a zone of about 60 miles for any first-class letter deposited by the last posted collection time. Two-day delivery is promised within a 600-mile zone, and three-day delivery is promised for distances over 600 miles. An accounting firm conducted an independent audit by "seeding" the mail with letters and recording on-time delivery rates for these letters. Suppose that the results of the study were as follows:

Use the given information to estimate the probability of an on-time delivery in New York.

Letter Mailed To Number of Letters Mailed Number of Letters Arriving on Time Los Angeles 650 525 New York 550 415 Washington, D.C. 450 405 Nationwide 7,000 6,220

Explanation / Answer

Probability of on time delivery to new York is given by

Ratio of no of letters arriving on time to number of letters mailed=415/550 = 83/110 = 0.7545

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