The mean one-way commute to work is 25.8 minutes. The longest one-way travel time is in New York City, where the mean time is 38.9 minutes. Assume the distribution of travel times in New York City follow the normal probability distribution and the standard deviation is 7.9 minutes.

A: What percent of New York City commutes for less than 30 minutes?
B: What percent are between 30 and 34 minutes?
C: What percent are between 30 and 41 minutes?

1 answer

A: 30 minutes is ... [(38.9 - 30) / 7.9] s.d. below the mean
... use a z-score table to find the percentage

B: 34 minutes is ... [(38.9 - 34) / 7.9] s.d. below the mean
... use a z-score table to find the percentage
... find the difference in the percentage from A:

A: 41 minutes is ... [(41 - 38.9) / 7.9] s.d. above the mean
... use a z-score table to find the percentage
... find the difference in the percentage from A: