Use the Empirical Rule. The mean speed of a sample of vehicles along a stretch of highway is miles per​ hour, with a standard deviation of miles per hour. Estimate the percent of vehicles whose speeds are between miles per hour and miles per hour.​ (Assume the data set has a​ bell-shaped distribution.)

About 68%

Given : The mean speed of a sample of vehicles along a stretch of highway is 66 miles per​ hour, with a standard deviation of 4 miles per hour.

i.e.  

We assume the data set has a​ bell-shaped distribution (i.e. Normal distribution).

To find :  The percent of vehicles whose speeds are between 62 miles per hour and 70 miles per hour.

i.e . The percent of vehicles whose speeds are between  miles per hour and  miles per hour.

i.e . The percent of vehicles whose speeds are between  miles per hour and  miles per hour.

i.e. i.e . The percent of vehicles whose speeds are within one standard deviation from the mean.

According to the Empirical rule , about 68% of the population lies within one standard deviation of the mean.

It means , about 68% of vehicles lies within one standard deviation of the mean.

i.e . About 68% of vehicles whose speeds are between  miles per hour and  miles per hour.

i.e .  About 68% of vehicles whose speeds are between  miles per hour and  miles per hour.

⇒ About 68% of vehicles whose speeds are between 62 miles per hour and 70 miles per hour.