Final answer:
Approximately 68% of cars are sold between 18 and 24 days on the car lot, as this range represents one standard deviation from the mean on either side in a normal distribution.
Step-by-step explanation:
The student is asking about the percent of cars sold between 18 and 24 days when the mean stay is 21 days with a standard deviation of 3 days, and the distribution is normal. To find this percentage, we use the properties of the normal distribution. The number of standard deviations from the mean for 18 days is (18-21)/3 = -1, and for 24 days it is (24-21)/3 = 1. These values correspond to the 16th percentile and the 84th percentile, respectively, because one standard deviation from the mean in a normal distribution captures 68% of the data (34% on each side of the mean). The percentage of cars sold between 18 and 24 days is therefore approximately 68%.