To calculate Kerry's average speed, use the formula:
\[ \text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}} \]
In this case, the total distance is 180 miles and the total time is 6.0 hours. Plugging in the values:
\[ \text{Average Speed} = \frac{180 \text{ miles}}{6.0 \text{ hours}} = 30 \text{ miles per hour} \]
Therefore, Kerry's average speed was 30 miles per hour.